J.G.
I heating pad really might help. I found with mine, the problem wasn't actually the mattress, it was whether or not they were in a deep sleep. If you try to move them when they are not in a deep sleep, there's a good chance they will wake. Look for signs she's in a deep sleep: limbs are limp (I usually picked up an arm to see if it would flop down), breathing has become very quiet and slow, no eye movement (not in REM sleep). I know there are other signs, those are just the ones I remember off the top of my head. Actually, the limp limbs was the one I always used.
Just a thought. Hope it helps.