I had the same troubles with my gelding. Honestly, you need to get a crop and beat them until they jump it. With this problem I would probably start with smaller jumps that they could jump from a standstill. I know this sounds kind of harsh, but you need to set the ground rules for your horse. If that doesn't work then I would unfortunately suggest getting a more experience or higher leveled rider to "train" your horse. With my gelding I had to do both. It sucks but is totally worth it.