Horse setting jaw on contact?

[Flibble]

I am guessing here but my new horse is lovely Irish cob hunter but when I take any contact he seems to lock his jaw. He doesn't lean it's like holding a brick not a sponge.
Bit is a NS verbindend one I had that was wide enough we have only been together 3 weeks and I tried hin in a loose ring with a bead n the middle and an egg but snaffle.

 

[charlie76]

I have one that does this. I put him in a fairly tight mexican grackle with a KK ported training bit. Once he started showing respect for this I changed him to an eggbut snaffle but still with the grackle.
If we do dressage he is in a tight drop noseband. Once he learnt ( with the training bit and grackle) that he could not grab hold and set himself he stopped doing it in most bits.
And cannot ride him in a cavesson though as he starts again.
If we have any lapses he goes straight back in the ported bit for a few sessions to remind him.