maybe radio interference (cordless telephone? wireless kb/mouse?), maybe bad driver, maybe antenna not firmly connected to router, maybe nm is seeing and trying to connect to different network for which it doesn't have the key (maybe your neighbour has same router and same default essid). Try in terminal
and you can see if there are any other networks nearby, their signal strengths, essid, encryption method etc.
you need to provide some more details otherwise you'll get very generalised shot-in-the-dark answers like this one.