Forms :: How To Force Execution Of A Query When Changing Record

I have the following tables:

Supplier
Supplier# [Other columns]

Material
Supplier# Material# [Other columns]

DeliveryHeader
Delivery# Supplier# [Other columns]

DeliveryDetail
Delivery# Supplier# Material# [Other columns]

I've created a form based on table DeliveryHeader with a subform based on table DeliveryDetail. The two are linked by Delivery# Supplier#.

On the form, Supplier# is a combobox that lists all suppliers from table Supplier.
On the subform, Material# is a combobox that lists only the materials supplied by the supplier selected with form's Supplier#.

The problem is that this second list always shows the materials of the first listed supplier. For instance, when the form is loaded, it shows delivery #1 with all its details on the subform. Suppose supplier loaded to form is #1.

1) If I use subform combobox, it shows correctly supplier's #1 materials. But then, whatever delivery I navigate, it always shows supplier's #1 materials.

2) If I navigate to another delivery without using subform combobox, suppose I stop on a delivery where the supplier is #4, then subform combobox shows correctly supplier's #4 materials. But then, once again, whatever delivery I navigate, it always shows supplier's #4 materials.

I've spread Me.Requery here and there but without any success. What trick must I apply to force the execution of the query of the subform combobox Material# each time I navigate to a new record on the form?