Problem with sit at class

Sigh, Rupert does not want to sit during class at all. Anything else I want him to do he's usually willing and able, it's just the sit. Anywhere else his butt hits the floor immediately so it's not that he doesn't know the command. I wondered about the floor but it's the same sort of floor as at our vets and he sits there no problem. And while we're waiting for it to begin and during the break he sits on the floor at class no problem. It's literally just during the class itself. Luring him doesn't work, rewarding him for sitting isn't making a difference and I can't figure it out. We've been going for about 3 months now and he even did a down stay on his back (his choice of position) this week so I don't think it's that class is overly stressful.

Does anyone have any ideas why I'm having this problem and/or what I can do to fix it?

A little info, don't know what's relevant but I'll throw it out there. He's around 4 years old, intact, I've had him almost 3 years, he's dog aggressive although mostly non reactive at class and has a very short attention span and generally acts a lot like a young pup except for the dog aggression. He's been clicker trained but the class is a more traditional one. He's happy and bouncy there while at the positive one we tried first he cowered and his tail was tucked so I'd say for him this is the better choice although I do still teach new things with the clicker at home.

Sorry for the length of the post and thanks for any advice or insights
Sarah

Very hard to tell as we canlt see, but a few thoughts.

Are you using the same body language and voice tone? as if the classes are different, one being more traditional, IME they tend to be really quite different with handling techniques etc.

uring the class, not sure what you do but perhaps you are asking him to sit whilst you are on the move ... it could just tip the balance so that he doesn't feel quite comfortable/safe on the floor esp. with lots of dogs. Is he comfy with the dogs or just being quiet?

I'd suggest trying Pawwax, see if that helps him feel more stabilised, if this is the problem. Another thing you could try is to take along a piece of non slip matting or carpet and see if he will sit on that.

Did he sit in the other class, the clicker one?

If it continues, regardless of the fact that he's sitting at some times and not others, I'd consider a vet check over.
I suspect he's just not feeling entirely comfortable either physically or mentally.

I don't know about the tone of voice and body language to be honest. Doesn't matter how I say it or what I'm doing anywhere else, he'll sit on command. Yes, he's being asked to sit on the move, same as when we're out on walks which he doesn't have a problem with. He ignores most of the dogs there now and is friendly with a few of them.

He didn't do a thing at the other class, cowered and shook but that was it. He has to go to the vet once a month to be weighed as he's underweight and he gets a quick check over then. Anything in particular we should look for?

We do have issues with attention which don't help us. I've worked at it but it's like he's got ADD or something. I'm getting a behaviourist in when I come back off holiday about that to see if they can help. And I'll ask about this problem then.
Thanks for the advice.