i also came across with this problem and finally i'd found out why model form appearing behind it.....
the solution try not to put setfocus in lostfocus method of any text box control because user will click other button in the form which call another model form, so the model form will appear behind it and the active form cannot release......



I've just posted a similar question, then thought I'd have another Search and your message on this looks very familiar.
I've also got a modal form with a new one appearing behind it; and then something odd happening where I cannot release an active form, even though nothing is visible which would stop this.
All very odd. I was blaming Vista and the issues with redraw, but then I saw it on XP and now I'm at a loss.