My TB gelding is 14, and very nicely mannered. However, over the past few weeks, he won't take the bit. I did change the bit several months ago, from a simple snaffle to a french link. But he has gone well on the new bit and isn't tossing his head or anything. He's eating fine. No teeth problems.
Once I get the bit in, he's fine. And I can get the bit right up to his teeth, and tickle around his gums, but he won't budge. He leans into me for a bit of a cuddle, I think, but simply won't open his mouth.