Maybe your horse is starting to get bored so doesn’t really try. At shows since it’s a different environment and more competitive your horse can sense the difference. You could try doing something different going out on a hack, riding in a different place, doing different riding work.
Use a whip or just a good leg and start your session with active work, either a medium trot or even straight to a faster/medium canter. Lots of transitions alllll the time!!! Circles, laterals. Constanatly change it up.
Trot poles too.
And if you can, trails etc. go for a run.