Windsuring guru Guy Cribb has a formula of 1/3 from the front of the sail back towards the clew (oljet?). Google ‘Guy Cribb harness lines’ and you’ll find it quick smart.
I just move them forwards or backwards until the sail feels balanced (doesn’t pull on either front or back hand) when up to speed and planing.
In your case, as you’re not getting onto the plane (when in the straps), a couple of things could be happening . Either there isn’t enough wind to get planing, or you are putting too much weight towards the back of the board too fast; in effect ‘putting on the brakes’. Be more gradual/gentle when going back towards the straps, make sure you don’t sheet out when thinking about where to put your feet and even try to maintain mastfoot pressure (hanging down on the boom with harness or hands transferring weight down through the mast). Or it could be other stuff, but that’s my first guess.
Edit: I see I was beaten to it.