Hidden Text in a list box

Hi, im a yr 12 IT student and ive been having some troubles with a list box
ive created. i've made a custom dialog box to replace a simple parameter
query which is working fine linking the form to my query, however when i
click the drop down list none of the text is showing. The scroll bars for the
drop down list is there, and when i rollover the list box its highlighted as
if there is text there, but there is not. When i click something in the text
box it correctly links the form to my query, but the problem is i dont no
what i have selected because i cant see the text in the list.

Please Help!