Perhaps you need a solid obstacle, like a tree limb. Some horses don't take a jump seriously until they cannot knock it over and quit. When CA created his back-40 training acreage, you can see that EVERYTHING is solid, from the open ended corrals to the cavaletti.
You are using, pardon me, flimsy equipment for jumps. My 4 cavaletti rotted out, but I liked that my horse would be punished if he hit the wood, even if it rolled shorted.