You're probably just getting tired after the first couple times. Ab strength can really help in back walk overs - leg strength is important, but once your legs reach a certain point you need to pull them the rest of the way over with your abs. Can you stand up from a back-bend? That uses your abs! Or just do some ab workouts - crunches, planks, etc. It'll help you with everything-jumps, body control, tumbling - so abs are really important!
I'm assuming the jumping back question is about a backhandspring? I've never heard of jumping into a back walk over. It takes a while to get used to it if you're just starting - going backwards is scary! Have you been doing it for a while? If so, do you have an octagon mat? It helped me in my backhandspring b/c you basically have to curve with the mat, so if you don't, you'll just roll until you do touch the ground... and if you practice your body will get used to it - muscle memory!
