Update problems: Tabular form with subforms

I have a main-subform, where the main contains lines from table 456 'Posted Approval Entry' and 2 subforms showing the header and the lines from the purchasing document. There seems to be some kind of updating problem: When I put a filter on the records in the main table so that the main-form is empty, the 2 subforms will not "blanked" but stil show the last record??