Thanks roger. That board has a mast track and i had it all the way back but it was still about 6-8 inches forward of the centerboard.
I have a 0.8, 1.2 and 1.6 m sails that i can try. Perhaps that was the issue. Too little wind for the 0.8m.
I imagine this problem would get worse with even smaller kids eg my 5 year old.