I need help on a round off flip flop?

I have a good back handspring and roundoff rebound. but when I connect it, it looks like crap. I bend my knees, almost like I'm sitting into the flip flop. when I do the flip flop, my back is all bent and so are my legs. if I rebound into the flip flop I undercut..well I undercut either way but more when I rebound. How do I fix that, and get my legs straight?