help needed with a combo box

Hi, I am trying to create a database for ordering materials. I have a form
with two tabs. The first tab is for order details and the second tab is for
order items (with a subfrm to list products for that order).
On tab one I select the supplier for the order. When I go to tab two, to fill
out the order items I have a combo box for ‘Product code’ and I want the
combo box to only show products for the supplier I selected on tab one.
The products are all in the same table and the column for ‘supplier name’ is
linked to ‘supplier name’ in another table for suppliers details.
In the combo box I have tried to create a query to only show products for
that supplier but it does not work for each record, It only works for one
record and does not seem to change when I change the supplier on tab one. I
also tried to set the criteria as ‘please enter supplier name’ (this is not
ideal put I did not know what else to try), but this works only once. It
works for the first product I enter and that’s it, it does not change until I
close and re open.

I hope this makes sense, and hope somebody can help.
Many Thanks, Alan.