can't choose value in combo box limited by the value of a text box

[vjb]

I have a combo box whose row source property is a query with two joined tables. The criteria is the value of a text box on the same form.
The values are listed correctly in the combo box based on the value in the text box BUT I can't choose a value from the combo box list. Only the first value in the list is shown in the combo box even when I click on another value.
Any ideas?
Thanks!!!!