[AccessD] Problem with filter by form

I'm quite inexperienced with filter-by-form so there may be a simple reason
why I'm not getting the desired results. Here is the problem.

Two tables, Regions and Districts. Districts are a child of Regions. On the
real form, the afterUpdate event of Regions does a Requery that uses a
static function to determine the current Region, and then display only the
districts in the selected region. This all works very nicely but not in the
filter-by-form scenario. The districts list seems to be frozen and not
amenable to the same manipulation that occurs in the ordinary
browse/view/edit form.

Anyone have any ideas why this might be so, and how I might work around it?

TIA,
Arthur