You may find yourself having white ink stick to your screen, while your other inks print just fine. White is the thickest of the inks. If you raise your off contact a little bit, that should help ease the problem. Because white is so much thicker, it may be that the thickness of the deposit is actually the distance of your off-contact. A little bit more should help the screen release easier.
Also let the platens cool a little bit between flashes. A hot platen can hang onto a screen.