S.F.
I also started having the same problem right around the time my daughter was that age. I determined that it was just that she was sleeping up on her knees or moving to the outter part of the crib that is harder to detect. Since I figured she was pretty much getting to the point where I wasn't so worried about SIDS anymore, I just started using the sound part from that point on. I figured it wasn't worth the constant interuptions to both her sleep and ours. It was definitely the best piece of mind for the beginning months though!