Whenever I do calf raises, whether they are on one of the standing, sitting, or 45 degree leg press machines, I have a problem where after each rep, my feet will slide a little bit down on the platform. After doing about 3-4 reps, they are about to slide off so I have to pause and reposition my feet further up on the platform before I can do another rep. Does anyone else have this problem? What do I do about it? It is really interfering with my calf exercises.