[AccessD] Tabbed form problem

I have a tabbed form with several pages containing subforms, and one or two
containing sub-subforms. Everything used to work fine but now suddenly when
I open the form, one or more of the tab-pages is blank. It's not consistent
either: sometimes only the first page is visible and all the others blank;
other times all pages but the last one are visible.

In design mode, everything appears normal. I even checked the visible
properties on the forms and they are all set to Visible.

I'm baffled. Any ideas, anyone?

Arthur