Bluetooth-connection; can't send strings prope

I have troubles with my program when i need to send Strings from my server bluetooth-socket to my client bluetooth-socket. Everything works fine as long as I am only sending one String at a time (for example chatting) but if I need to write more Strings at a short period of time (to interchange informations), the Strings will not get seperated from the client code. For example if I'm sending "FirstUser" and right after that "SecondUser" the client does not read "FirstUser" and then "SecondUser". It will read "FirstUserSecondUser". How can I avoid this behaviour?

Edit: If I let the Thread sleep before it is able to send a new message, it reads the right strings but this solution is not working fine for my need.

Server-Code: sending to all clients(edited)

   public synchronized void sendToAll(String message)
{   
    try {
        Thread.sleep(100);
    } catch (InterruptedException e1) {
        e1.printStackTrace();
    }

    publishProgress(message);
    for(OutputStream writer:outputList) {
        try {
            writer.write(message.getBytes());
            writer.flush();
        } catch (IOException e) {
            System.out.println("Some-Error-Code");
        }
    }
}

Server-Code: reading from a client:

   public void run() {
    String nachricht;
    int numRead;
    byte[] buffer = new byte[1024];
    while (runningFlag) 
    {
        try {
            if((numRead = inputStream.read(buffer)) >= 0) { 
                nachricht = new String(buffer, 0, numRead);
                serverThread.handleMessage(nachricht); 
            }
            }
             catch (IOException e) {
                this.cancel();
                e.printStackTrace();
            }
    } 
}

Client-Code: reading from server(edited)

@Override
    protected Void doInBackground(Integer... ints) {    
        String nachricht = new String();
        byte[] buffer = new byte[1024];
        int numRead;
        while (runningFlag) 
        {
            try {
                if(((numRead = inputStream.read(buffer)) >= 0)) { 
                    nachricht = new String(buffer, 0, numRead);
                    publishProgress(nachricht);
                }
            }
             catch (IOException e) {
                clientGame.finish();
                e.printStackTrace();
            }                      
        }
        return null;
}

Client-Code: writing to server

public synchronized void write(String nachricht)
    {
        try {
            Thread.sleep(100);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

        try {
            outputStream.write(nachricht.getBytes());
            outputStream.flush();
        } catch (IOException e) {
            this.cancel();
            e.printStackTrace();
        }
    }

I appreciate every little help :) .

You need to encapsulate your data item to avoid concatenation. It means that you have to write and read a whole data item before continuing.

You should have some utility methods to do that instead of directly using methods of OutputStream and InputStream :

public static void writeItem(OutputStream out, String s) throws IOException
{
    // Get the array of bytes for the string item:
    byte[] bs = s.getBytes(); // as bytes
    // Encapsulate by sending first the total length on 4 bytes :
    //   - bits 7..0 of length
    out.write(bs.length);      // modulo 256 done by write method
    //   - bits 15..8 of length
    out.write(bs.length>>>8);  // modulo 256 done by write method
    //   - bits 23..16 of length
    out.write(bs.length>>>16); // modulo 256 done by write method
    //   - bits 31..24 of length
    out.write(bs.length>>>24); // modulo 256 done by write method
    // Write the array content now:
    out.write(bs); // Send the bytes
    out.flush();
}

public static String readItem(InputStream in) throws IOException
{
    // first, read the total length on 4 bytes
    //  - if first byte is missing, end of stream reached
    int len = in.read(); // 1 byte
    if (len<0) throw new IOException("end of stream");
    //  - the other 3 bytes of length are mandatory
    for(int i=1;i<4;i++) // need 3 more bytes:
    {
        int n = in.read();
        if (n<0) throw new IOException("partial data");
        len |= n << (i<<3); // shift by 8,16,24
    }
    // Create the array to receive len bytes:
    byte[] bs = new byte[len];
    // Read the len bytes into the created array
    int ofs = 0;
    while (len>0) // while there is some byte to read
    {
        int n = in.read(bs, ofs, len); // number of bytes actually read
        if (n<0) throw new IOException("partial data");
        ofs += n; // update offset
        len -= n; // update remaining number of bytes to read
    }
    // Transform bytes into String item:
    return new String(bs);
}

Then you use these methods both for server & client to read and write your String items.