I'll take a stab at this...

Probably the last setting for the form was a minimized state; therefore, when you select restore it automatically minimizes the window.

Try this:

1. Maximize the form.
2. Select the Form Caption/Title bar (whatever you want to call it) and move the form.
3. Resize the form accordingly.
4. Press the maximize button.
5. Press the restore button. Should restore to the size set in #3.

Did this work? If not, sorry to waste your time.