Hi Gemax,
Are you planing when you try to put your foot in the footstrap? You'll know when you're planing, because suddenly you're going twice as fast as you ever have gone before, and the board skips along and becomes very very twitchy. It also makes a noise like a machine gun!
You definitely need to be planing when you go for the footstraps. If you're not planing, there's no point going for the straps, as you just sinnk the tail and then stop.
Assuming you're on the plane, you need to push down with your toes when you put your foot in the strap. This means that the windward rail doesn't sink, and you keep going in a straight line. After a bit of practice it all becomes easy and you wonder what all the fuss was about