At home my horse is really good at jumping but when I go to shows she doesn't jump at all

Get some schooling classes in
try taking her into some flat classes first, eventually she will notice how off property is a new enviornment and how its okay. start off in the lower vertical or crossrail divisions or try to bring her to small barn shows or clinics, then start working your way up
This may be because she is in a new environment and gets nervous
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