Replying to Boot bang

So i rode with Solomon Koas SPK's last year. Now I didnt really do anything other than park and groomers in these boots but ran into one major annnoying problem. When i would come down on a landing or go from groomed snow to slush my entire foot would slide up in the boot and hit the end. Now i tried tightning the buckles (both of them) and the power strap but still ran into the same problem. Now if theres something i can do to fix this i'd love to know about it, but if this is a reacurring problem i was thinking about getting a pair of full tilt hotdoggers. Any advice?