select menu no open

select menu no open

Hi I want show a select menu , this work the first time that I open, but when I want open again, it doesn't open
this is my code:

<form action="#" method="get">
                <div id='divEspecialidad'>
                    <label for='especialidad' class='select'>Especialidad:</label>
                    <select name="especialidad" id="especialidad" data-mini="true"  >
                           <option value="">Todos</option>
                    </select>                   
                </div>
            </form>

I populate this select with other elements via javascript.
I don't know why about it..
Help pls