Well, read all responses, but I disagree. I used to think that my horse was showing me up, when I'd take him somewhere different & try to show off what he 'knew'. Then I started studying animal behaviour & psychology... & learned what he really 'knew'!
Horses(& animals generally) aren't able to generallise very well. They can learn well how to respond to something in some environment, but that doesn't equate to them 'knowing' it if the cue or tool is slightly different, or it's in a different environment. You need to actively teach them in a variety of ways & environments for them to truly begin to understand that x=x regardless of circumstances.
I would start by teaching the horse as if from scratch in a variety of settings. However, it shouldn't take him long to pick things up, if he's already learned them well before.