Seems to me the problem might come from a lack of shoulder and/or lower back flexibility. Without the proper strength and flexibility, it is difficult to do. Keep praticing the back walkover. Doing back walkover type skills and exercises without having good shoulder flexibility can cause a lot of stress on the lower back. Start working on some shoulder flexibility. A lot of the kick over part comes from the shoulder not just the legs. Make sure you think about that when doing the skill.
Heres some shoulder flexibility.
-Lay on stomach and put hands behind your head. Have someone pull your elbows together as far as they can go hold for 30-45 seconds.
-Still laying on stomach put your arms down by your sides. Have someone pull your arms up as far as they can go. hold for 30-45 seconds.
-not put your arms above you have someone pull your arms up as far as they can go without removing your forehead from the floor. hold for 30-45 seconds.
-Put your hands on a medium or high beam and hand someone push on your shoulders for 30-35 seconds.
-get into a bridge, push shoulders over and past hands...try to go your legs straight.
thats just some stuff you can do everyday...i'm sure your coach could give you more....also try do do some back strength as well.
Good luck and hope it works out.