Your straps just may need to be pulled in tighter around your ankle maybe? If you pull them in as tight as possible but still comfortable this might work. If you pull them tight but they don't stay clipped in then you will need new clips or replace the bindings. When bcoking for a while the straps will tend to loosen on their own cause of the type of extreme movement that comes with bocking.
Try to wear some high-cut shoes when bocking as this will take alot of the pressure from the strap and make it more comfortable
Also you can modify the straps by replacing them with some bindings used for snow boards. They are wider and can be alot more comfortable. They will cost a few dollars though but there is a sonw shop in The City that sells single strap bindings instead of buying the whole set up