Subform does not refresh

A form and a subform are properly linked through master/child fields.
On the main form there is a combo drop down list based on "select
distinct" query. When I select the value from that list, the subform
does not display associated records, yet when I place the mouse pointer
into the drop down list (without opening it), and roll the scroll wheel
on my mouse (the one between left and right buttons), the values in the
combo box change and associated records in the subform are all
displayed correctly. Why doesn't the subform update when I select the
value from the drop down list?