Replying to Weird problem with binding

So this problem has recently been annoying me.

I have salomon STH 12 Drivers mounted on some rossignol skis, and at the beginning of the day when i click in to the bindings they are perfect and nice and tight. But by the middle of the day I find myself being able to shake my foot back and forth and my boot slides around in the binding very slightly. What I have been doing is moving the heel piece forward very slightly each time this happens, but it always will happen again the next day.

This actually has happened on another one of my binding which are fairly old pivot 12's. So That leads me to believe that it is not the problem of the binding.

So my question is is there something that I could be doing while skiing, or something wrong with my boot that could be making this happen? Should I just go to a shop and ask them or am i good doing what I am now?

Thanks for any help that you can give.