I have 2 suggestions, one is pretty "off the wall" but it works well for me in powered up and overpowered conditions, just not so good when there's barely enough wind to waterstart.
Instead of putting my back leg or foot up on the board, I stay in the water so I can move in really close to the tail of the board.
Then really stretch your arms up tall and kinda "throw" the rig to give it some inertia.
Let the rig pull you up from the water and land you on the board (almost anywhere on the back of the board will do) then sheet out momentarily, get your feet in the right places and sheet in/sail off.
Works for me, as I'm pretty short (5'6") and I guess I'm not all that "flexible".
If I put a foot on the back of the board I fall back and get water up my nose/my head wet/ push the back of the board away/ etc. so I learned to waterstart this alternative way.
The 2nd suggestion is similar but has to do with the technique you are using (back foot on the board).
Try to stay low and then when you are sure you are ready, "throw" the rig up to give it as much inertia as possible, kick you feet, any thing you can think of to "launch" your body up and over the tail of the board.
Are you sure you are extending your arms as long as possible when flying your sail and just at the moment you want to launch out of the water?
Hope this helps,