This is a common problem. Check a few things to see what the problem is. First make sure the round off is long and low to the ground with the lunge lever and turn all moving forward. The toe in the front lunge leg must be directly under the knee and the lunge should be as long as your leg. Your arms should be by the ears the whole skill – smell your armpit during the round off. make sure the second arm is turned backwards to initiate the block. To block shrug through your shoulders and push your hands as if you were pushing the mat out of the way. Make sure there is a straight line from Hands to back foot throughout the whole round off. The legs must come together near the top of the round off to get a quick back handspring which is stretched a lot through the shoulders to produce a sling shot effect out of the handspring so you have adequate height for the double. It is rare for the problem to be in the double unless you are not executing a layout first then twisting. Most of the time it the round off back handspring that causes the problems.