They're fussy rascals to be sure.
Are you trying to line up the beginner jumps or the advanced jumps?
If you're attempting to line of the advanced jumps, and presuming you're going to use two rows of jumps, try placing them parallel rather then sequentially. In other words, place the first two jumps in each row, then place the second two jumps, and finally the third pair of jumps. Also, make dead sure there's nothing in the way, like a vegetable and ensure your horse is not engaged with any of the jumps. If any Sim is in the process of making a jump you won't be able to position subsequent jumps.
Beyond that the best advice I can give is to be patient and stay with it. There's much about the Pets expansion that's not ready for prime time, placing the jumps is one of them. I think it would have been better to plot the entire course off a cornerstone jump. Once that cornerstone is placed the course is laid out and we simply drop the others onto the designated spots.