A horse will usually refuse for a couple of reasons. 1) the horse is unconfident, 2) the horse is sour on jumping, 3) the horse is in pain, 4) the rider is getting in the way (rider error).
So first make sure your horse is not in pain. Next, make sure you are balanced, off her front end and not getting in her way. It doesn't sound like your horse is sour, so it might be an unconfidence issue.
I'd go back on the ground and make sure she is confident trotting over poles, raised ground poles, then eventually jumps. Do this on-line. When you know FOR SURE that she is confident you can then try it riding. If she refuses the WORST thing you could do is smack her. If she refuses and you smack her, why on earth would she ever want to jump for you? She won't. The reason I say this is because I see so many riders smack their horses and try to force them over the jump when they refuse. It ruins the relationship for one, not to mention it just proves to your horse that you can't be trusted.
Okay, now that my mini rant is over lol.....if she refuses then walk her away and then walk her back toward the jump and let her investigate. Once she is relaxed walk her away and re-approach. Do that until she isn't even interested in the jump.
Also, when working her on the ground and while riding, does she maintain gait when asked, or do you have to keep asking her to go?