Hi dan
I had the same problem on my old board even after i spent $70 on new straps .
IMO I think that it has more to do with the pads and angle of ones foot on the board than the straps. I was riding a Naish TTpro and the pads are flat, hard and $hit .But after some playing in the shed I solved the problem and in the process probably invented the best strap around.
I found that the material in the straps does not have enough elasticity so my feet kept coming out .
So i inserted two small pieces of foam/rubber under the neoprene on either side of each strap which conpresses when i insert my foot .You could stick the foam to the existing foam with (Selly kwik grip contact adhesive ) but i have not done this get . work amasingly well and is still posible to kick your board off if the $hit hits the fan. And cost about $2
send me an email if you want more info .