Tesseract empty page

I use tesseract for detecting characters on image.

        try
        {
            using (var engine = new TesseractEngine(@"C:\Users\ea\Documents\Visual Studio 2015\Projects\ocrtTest", "eng", EngineMode.Default))
            {
                using (var img = Pix.LoadFromFile(testImagePath))
                {
                    Bitmap src = (Bitmap)Image.FromFile(testImagePath);
                    using (var page = engine.Process(img))
                    {
                        var text = page.GetHOCRText(1);
                        File.WriteAllText("test.html", text);
                        //Console.WriteLine("Text: {0}", text);
                        //Console.WriteLine("Mean confidence: {0}", page.GetMeanConfidence());

                        int p = 0;
                        int l = 0;
                        int w = 0;
                        int s = 0;
                        int counter = 0;
                        using (var iter = page.GetIterator())
                        {
                            iter.Begin();
                            do
                            {
                                do
                                {
                                    do
                                    {
                                        do
                                        {
                                            do
                                            {
                                                //if (iter.IsAtBeginningOf(PageIteratorLevel.Block))
                                                //{
                                                //    logger.Log("New block");
                                                //}
                                                if (iter.IsAtBeginningOf(PageIteratorLevel.Para))
                                                {
                                                    p++;//counts paragraph
                                                    //logger.Log("New paragraph");
                                                }
                                                if (iter.IsAtBeginningOf(PageIteratorLevel.TextLine))
                                                {
                                                    l++;//count lines
                                                    //logger.Log("New line");
                                                }
                                                if (iter.IsAtBeginningOf(PageIteratorLevel.Word))
                                                {
                                                    w++;//count words
                                                    //logger.Log("New word");
                                                }
                                                s++;//count symbols
                                                //logger.Log(iter.GetText(PageIteratorLevel.Symbol));
                                                // get bounding box for symbol
                                                Rect symbolBounds;
                                                if (iter.TryGetBoundingBox(PageIteratorLevel.Symbol, out symbolBounds))
                                                {
                                                    Rectangle dueDateRectangle = new Rectangle(symbolBounds.X1, symbolBounds.Y1, symbolBounds.X2 - symbolBounds.X1, symbolBounds.Y2 - symbolBounds.Y1);
                                                    rect = dueDateRectangle;
                                                    PixelFormat format = src.PixelFormat;
                                                    Bitmap cloneBitmap = src.Clone(dueDateRectangle, format);
                                                    MemoryStream ms = new MemoryStream();
                                                    cloneBitmap.Save(ms, ImageFormat.Png);
                                                    ms.Position = 0;
                                                    Image i = Image.FromStream(ms);
                                                    //i.Save(ms,System.Drawing.Imaging.ImageFormat.Png);
                                                    i.Save("character" + counter + ".bmp", ImageFormat.Png);
                                                    counter++;
                                                }
                                            } while (iter.Next(PageIteratorLevel.Word, PageIteratorLevel.Symbol));
                                            // DO any word post processing here (e.g. group symbols by word)
                                        } while (iter.Next(PageIteratorLevel.TextLine, PageIteratorLevel.Word));
                                    } while (iter.Next(PageIteratorLevel.Para, PageIteratorLevel.TextLine));
                                } while (iter.Next(PageIteratorLevel.Block, PageIteratorLevel.Para));
                            } while (iter.Next(PageIteratorLevel.Block));
                        }
                        Console.WriteLine("Pragraphs = " + p);
                        Console.WriteLine("Lines = " + l);
                        Console.WriteLine("Words = " + w);
                        Console.WriteLine("Symbols = " + s);
                    }

And it works when I have an image with a lot of text, but when I have an image with only one letter it does not.

It found a symbol, I see it in input. Symbols = 1. But it cant get BoundingBox. Why? The same whem I use alphabet image

You may need to test the OCR with different page segmentation mode and OCR Engine mode to get the best result. Below is the usage information available in Tesseract 4.0.

Page segmentation modes:
  0    Orientation and script detection (OSD) only.
  1    Automatic page segmentation with OSD.
  2    Automatic page segmentation, but no OSD, or OCR.
  3    Fully automatic page segmentation, but no OSD. (Default)
  4    Assume a single column of text of variable sizes.
  5    Assume a single uniform block of vertically aligned text.
  6    Assume a single uniform block of text.
  7    Treat the image as a single text line.
  8    Treat the image as a single word.
  9    Treat the image as a single word in a circle.
 10    Treat the image as a single character.
 11    Sparse text. Find as much text as possible in no particular order.
 12    Sparse text with OSD.
 13    Raw line. Treat the image as a single text line,
                        bypassing hacks that are Tesseract-specific.<br>

OCR Engine modes:
  0    Original Tesseract only.
  1    Neural nets LSTM only.
  2    Tesseract + LSTM.
  3    Default, based on what is available.

For example,

psm 8 would give the best result for OCR a single word
psm 6 may give the best result of a block of text

In your code, it showed you have used the default engine mode and not specified segmentation mode. You may do some more tests to find out which modes give the correct result.