So I'm still continuing the experiment, I put substrate over the hotspot and this is the fourth feeding that almost all my ball pythons missed! So I've decided to go back to my original setup and move the substrate away from the hotspot, put the ball pythons directly on the bottom of the tub on the hotspot and 'nestle' them with Repitchip (none under the snake). I'm thinking that just that one inch of Reptichip under the snake kept them from getting warm enough on the hotspot and causing them to go off of feed.