If you are using the socket-clamps instead of bolt-down screws, slippage is normal. If you bolted it down and it still moves, I would probably break into a crying fit of 2-year old (or 80 year old, whichever) quality. It also depends on how much slipping is going on. Also, would you describe the slipping to be like... nascar oil-slick slippery... or merely old-lady icy sidewalk slippery? Okay, that wasn't necessary... but...
As long as you feel decent pressure on the die, and your temps are fine... slippage is normal. Just don't keep doing it or your AS3 won't cure properly. In fact, you might want to apply a new layer just in case you 'slipped' it off.