Bootstrap input group field not sizing correctly

I have a problem with Bootstrap. I'm kind of new to this framework, and I'm trying to get a searchbar with a <select> dropdown and a button to click on for searching. But for some reason, the <input> isn't not sizing up to the column width. It's staying a very small size but the dropdown and the button to the right of the <input> tag are where I want them to be. Could someone help me out?

Thanks!

<div class='container'>
  <div class='row'>
    <div class='col-sm-12 col-md-12 col-lg-12'>
      <form method='get' action='search_results.php' class='form-inline'>

        <div class='input-group'>
          <div class='input-group-btn'>
            <select id='selector' class='selectpicker form-control' data-live-search='true'>
              <option>Categories</option>
              <option>Computers</option>
            </select>
          </div>

          <div class='input-group'>
            <input type='text' class='form-control' placeholder='Search' name='searchTerm'>
          </div>
          <div class='input-group-btn'>
            <button class='btn btn-default'><i class='glyphicon glyphicon-search'></i>
            </button>
          </div>
        </div>
      </div>
      </form>

    </div>
  </div>
</div>

What happened was you were wrapping everything in input-group, which basically controls the sizing of the object. I removed the wrapping around the search, but kept it for the category. You can see I set an inline css of 150px width (you can set this on the CSS side to better manage it)

<div class='col-sm-12 col-md-12 col-lg-12'> 
                <form method='get' action='search_results.php'>
                    <div class='input-group'>
                        <div class='input-group-btn' style="width:150px">
                            <select id='selector' class='selectpicker form-control' data-live-search='true'>
                                <option>Categories</option>
                                <option>Computers</option>
                            </select>
                        </div>
                        <input type='text' class='form-control' placeholder='Search' name='searchTerm'>
                        <div class='input-group-btn'>
                            <button class='btn btn-default'><i class='glyphicon glyphicon-search'></i></button>
                        </div>
                    </div>
                </form>
            </div>