When you are using a select list to set values for an add to cart you must ensure that the value you want to add to the cart is set in the value attribute of the options inside of the select list like this:
<select name="test">
<option value="1">1</option>
</select>
It is what is in the value that will be posted when the form is submitted. The other thing you need to check is that the select list is inside of the add to cart form, if it is not the values won't be added to the cart. When you are using a select or multi select list you will only be able to submit a single value to the cart, not multiple.