This issue is that a gallery will not display items matching a combo box value until a user interacts with the combo box. This is because even if you set the DefaultSelectedItems to something, the gallery filter will not recognize the item displayed int the combobox as "selected" and the gallery will be blank until the user interacts with the combobox. This was confirmed by a community support team member here.
This makes it difficult to pass the variables and parameters to the combo boxes on various screens and have galleries show the expected items. Each screen has to have the user reselect his category again and again (or use hokey work-arounds that don't work very well).