You have my sympathy,DD1 and dpony sometimes get in a vicious circle like this. DD1 thinks dpony will refuse and so doesn't kick on,dpony who is nobody's fool,picks up on the hesitation and then does refuse.At home and in lessons they fly around the field,jumping everything,but at shows and sometimes PC rallies,DD1 often ends up in tears because dpony has decided that she just isn't going to play.
At the last show,DD1 had a horrid time in the 2ft show jumping,dpony refused one jump about 3 times.DD1 left the ring in tears,DD2 jumped on dpony and got clear until the last jump,which Dpony refused.However DD2 said it was her fault as she hadn't gone in at the right angle.Poor DD1 had to watch her sister,who had only ridden dpony 2 times in the past 6 months do far better than she did.
Can your DD ride your other pony for a while,so they have a break from each other?
What is he like ridden bareback? That might point to a saddle problem or not.
Dpony reacts well to being whizzed up,or growled at before a jump.Would that help?
Can you set up some grids? We set up a row,I stand at the end holding up say,3 fingers and the DDs have to jump down the grid and tell me how many fingers I'm holding up.Stops them from looking down,they look over the jump,pony jumps over the jump,look down,pony gets message to stop.
DD1 swapped ponies with a friend at a rally today,and it was a real confidence booster for them both.
I hope that you can sort things out.It is so frustrating.