Redraw not working

My application has several windows and some of them does not redrawn properly after other windows obscured them. It happens on some of my PCs (Windows XP). (1 out of 10).
I draw the picture using front buffer only.
I think there is some ralations between my flush(glFlush()) and screen refresh rate.
If I put time dalay, it seems working fine.

Any idea for the problem will be very much appreciated.