If you have a button appended to an input control in Bootstrap, and you want it to fill the entire width, it’s not sufficient to add input-block-level to the input itself; this CSS class also needs to be added to the surrounding .input-append div. For example:
<div class="input-append input-block-level">
<input type="text" class="search-query input-block-level" name="q" placeholder="Search">
<button type="submit" class="btn btn-primary">Search</button>
</div>
Applying .input-block-level to only one of the elements (either the div or the input) just doesn’t work.