Can't say I've got the back flip down, but I do have the wall run, so here's what I think: When you do the wall run, you're trying to achieve height, so you drive your force upwards. When you're trying to do a trick like you're doing, however, you're trying to suspend your body in the air just long enough to land the back flip. If you watch the pros of parkour do this, you'll see they drive outwards, rather than height.

go at a angle up instead of pushing out