15 month old Dobermann. Very small, and athletic, problem is not that she can't jump, but that she does not see the logic in not touching it. Have only used a portable jump and she will literally smash through it. Tried it low first, but same problem. Did use the tug to try to bring her up and over, same problem. Now she is only rewarded with the tug when she is successful at not touching the jump.
Often times the distance started form the jumps can make a big difference. this is really important to horse folks. A long distance will cause a fast, flat jump. this also causes touching. Close up often makes the dog have to realy think about getting over it. Our trianing jump has a top board that spins if the dog hits it. Works for some dogs, others not. Also a rope streached across the jump about an inch above it can cause the dog to avoid contact. Just make sure it's not tight enough to trip the dog. Also ty placing a board on the ground the same distance away as the jump is high.
Quit trying for any kind of height until the dog jumps cleanly at shoulder height.
I agree about rewarding only for a clean jump.