[xmonad] inconsistent behaviour of mod+shift+return

i am a newbee with xmonad. The issue is that when i use mod+shift+return to open xerms, then sometimes (10% times)it works. But usually the screen flickers and no xterm opens. i have to press the m0d+shift+return multiple times,and then i get like 2 or 3 xterms opened. So its not very consistent. ps:I have not changed any default key bindings, everything else seems fine.