Forms :: Limiting Combobox With Query

I'm trying to filter the options in a combo box based on two criteria. The first criterion is another combo box on the same form, for a cascading combo effect. This criterion works fine, no problems at all. The second criterion is a query. So the combo box should display only those values that match both the upstream combobox AND the results of the query. What's really frustrating is that this combobox works just fine in my development dbase. It only stops working when I move it into my active dbase. Anyway, here's the SQL for the combobox rowsource:

SELECT DISTINCT Size2.SizeID, Size2.XYdim
FROM (Shapes INNER JOIN (qrySignsInInventory INNER JOIN Size2 ON qrySignsInInventory.SizeID = Size2.SizeID) ON Shapes.ShapeID = Size2.ShapeID) INNER JOIN Signs ON Shapes.ShapeID = Signs.ShapeID
WHERE (((Size2.SizeID)=[qrySignsInInventory].[SizeID]) AND ((Size2.ShapeID)=(SELECT Signs.ShapeID FROM Signs WHERE Signs.SignID = [Forms]![frmUsedDropdown]![cmbSignID])));

I've hilighted the criterion that isn't working. Also, I'm not getting any errors or anything, it's just that the results aren't being filtered. I've triple-checked the references for the active dbase, and I've checked the results of the query. Everything should be working.