This is the classic Syntax problem with Tabbed Subforms.
I always struggle with getting the correct format and sequence of the [Form] anf field names.
There is a simple way out of it if you can't find the correct sequence and that is to place a text field on the mainform and use the combo's after update event to place the combo choice in that Field.
Therefore you only have one form and field to refer to in the querys criteria.
if you could post the database with some dummy data in it I would try and get the sequence correct.