Replying to Question about setting the px12

I had a shop set my binding. However, it seems that the binding is set a little smaller. Each time i step in, the heel piece will slide backwards(i am guessing how they will release) to accomadate the boot.

Now my question is did my shop set the binding too small for the boot? If i move the binding back one notch, then the boot will go in perfectly without the heel piece moving backwards. Is the px12 binding supposed to be set so that the heel piece will slide back when you step in or should it be set so that it wont move when stepping in?