Paint Brush Is Not Fast Enough To Draw

Hi,
I am making a paint canvas application , i have only a problem i have used invalidate to refresh and repaint but in output when i move cursor slowly it paints perfectly but when i move cursor fast it gives me dots and there is gap between them which i don't want, can you tell me how to solve this problem?

Thanks for your time

code for invalidating is:

Me.Invalidate(objgraphicsitem.rectangle)