You need to tell us more about what's happening with the sail and board??? Are you just landing overpowered, hooked in and just being pulled over the front. Are you extending you back leg when you land so that you plant the tail???? Or are you crouched up in a ball when you land and just falling in a heap???
Lots of things could be going wrong, try Sheeting out to dump power on landing, your lines may be too short to sink your hips and control the power, Your straps could be too tight not enabling you to grip the board with your feet causing you to slip and stack???
Get someone to film you stacks so we can have a look