Did you try it the other way around? First hide the form, then enable the other one? When showing and hiding forms, I always show the form that I want to have focus last. just simply... (as an example)

Me.Hide
MainForm.Show

Now that I think about it though.. the problem may be that the form that you hid doesn't really loose focus because it's just hidden. I'm completely pulling this out of the air.. I don't have anything to base this on.. just a thought. I'd guess what you'd have is...

Me.Hide
MainForm.Enable

Try putting a "MainForm.Show" after that...