checkbox label with div in it on ipad not clickable

i have the following situation:

i have made hidden checkboxes and labels which are clickable.

when testing in browsers all works like a charm, but on ipad the label only take the click, when i am not hitting any text, which is in the label.

why is that so?

here my markup of one box:

<input name="Quantify-Studienzusammenfassung" value="no" type="hidden"  />                          
<input class="chk_button" name="Quantify-Studienzusammenfassung" id="chk_quantify_studien" type="checkbox" value="yes"  />
<label for="chk_quantify_studien">
    <div class="label_div">
        <h3>Quantify-Studienzusammenfassung:</h3>
        <p>Kurzatmigkeit vs. Tiotropium/Formoterol (in freier Kombination)</p>  
  </div>
</label>