Do you know why a horse who was amazing at getting on the bit can be now really hard in his mouth and not listening (nothing has changed with the bit) ?

Perhaps his teeth hurt, or his back, or his neck, or maybe your hand is too hard or you're not using your aids right.
Check his teeth or he maybe just bored or hurting
It could be his teeth, he/she could be bored, or they’re just seeing what they can get away with
On the bit does not mean head position. If you are riding your horse "on the bit" by holding it's head in position, there's a good chance that that is why you're having problems. Not saying that you do, but many people do and then wonder why the horse is starting to have issues.
It could be his teeth, it could be time to get them done
Sounds like he might be getting bored with work or if he is in quite hard work may need a spell to settle down again
My reacted the same way when his saddle needed to be checked again.
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