PS3 sensors aren't picking up side and body planks

For my past two workouts when I got near the end and started doing the side and body planks the sensors didn't seem to be picking up my movements. I would follow the on screen instructions, but my avatar would not move. There was not a message indicating a loss of connection. However, I removed the usb receiver, waited for the sensors to power down and reconnected everything, but the problem persisted. Oddly, when I got up to get the controller the system interpreted it as me doing the excercise. The sensors had been working for the whole workout prior to this (about 20 min). I was also able to do them without problem before my past two sessions.
This is on a PS3. I'm about 4 feet from the receiver and I've cleared any items that I expect would cause interference from the receiver.|||I had the same issue today with the "Straight Arm Plank" and half of one of the "Squat/Hold"|||I've also had problems with the side planks. Squat and hold I'm ok with, squats with calf raises not so much. That might be my fault though. Those are tough.