For some reason the subform only "sees" one record at a time. Each field in txtUD is being populated with whatever record has focus.
Me.txtUD = DSum("[UnitsOrdered]", "[Inventory Transactions]", "[Job]=" & Nz(Me.txtJob, 0) & " AND [PartNumber]=" & Nz(Me.txtPart, 0) & " AND [TransactionID]<=" &...