Tompkins' try, I'm with Nigel on that one. Some of you may recall a flippant piece I put up on the referees' thread some years ago re balls going down from the hands in front of a player. I suggested refs carry a bricky's plumb weight (that solid bronze piece used to ensure courses of bricks are straight and vertical) and a length of string to ascertain if a ball travelled forward or backward from a players' hands when dropped. It is absolutely crucial to determine the point of dropping when said ball was dropped, NOT where the hands were when the ball hit the ground, that's where the ref had to hold the piece of string after unfurling it and attaching the plumb weight. Impossible. Same with slo-mos, by the time the ball hits the ground the recalcitrant player's hands are some distance forward of the position where they were when the ball was dropped, suggesting the balls travelled backwards when most times it didn't.
The solution is to presume ALL balls dropped in front of a player are a knock-on. In fact I'd suggest this be incorporated in a revision to the laws. Use a bit of common sense, iRB/WR (World Rugby).