I am using this for select combo,After adding its css and Js files i got this result. Here what i have tried so far.
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<link href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.4/css/bootstrap-select.css"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.4/js/bootstrap-select.js"></script>
<select class="selectpicker show-tick form-control">
<option>pen</option>
<option>pencil</option>
<option selected>brush</option>
</select>
The output.
So can you please have a look at this why this is happening ?

this is because the sequence of loading css file not correct. Combo css file must load after loding all other css files then the issue will resovled. If still not resolve then css classes or properties are overriding somewhere.