Well one thing could be your tuck. Make sure when you jump up that you immediately tuck when your in the air. Throw your arms and chest into your knees. And what this will do is allow you to untuck faster so you can land it with a better extension of the legs.

Hey brother its all about experience you nee to get to use that..

Lots of reasons, if you post a video we can help you better.

You probably running too fast in the beginning when you're about to do your front flip or you're tucking really fast and that's going to help push you down what you want to do is when you land bend your knees to soften it and don't run up to your front look

Be sure to spot your landing. Use your arms to boost into the air and glide into your quick tuck. Come out of the tuck a little early and extend your legs and spot your landing.

don't let it knees buckle

Jump higher and practice practice practice

If you land the jump on your feet but then fall it might just be the trampoline causing imbalance through its unstable surface although I'm not sure what this has to do with a wall run.