Can't select Combo item

[SL]

I have an unbound combo box on a single form displaying depot names based on
a query. I want to select a depot name from the combo and then click a
command button to open a report based on the depot selected.

I can click the combo so the names drop down and they are all there but I
can't select any names from the box.

However, if I go to the form design view and then re-open the form I can
select from the combo and run the report no problems.

Does anyone know why the combo is only being 'activated' once the form has
been into design view & then re-opened?

Thanks

Sonya