I've had this problem with my French bowing recently. As I'm playing, my hand tends to gradually slip and move towards the tip. I even use a rubber grip. The three fingers on the frog tend to slide towards the tip and push the index finger until it comes off the grip. It causes me to have regrip the bow on the fly. Perhaps the momentum of my wrist motion gradually pulls the fingers with it. These fingers are not gripping, but just laid gently on the frog. It sounds like a stupid problem. But it's been bugging me and my teacher doesn't seem to have an answer. He claims my technique is fine. Is it normal to have to regrip the bow while playing? Is there something technique-wise that could counter this problem?