Anything. They don't get better at anything? I don't understand why this is a confusing question.
From what I have seen over 30 years of training, many people reach a certain level - doing the techniques pretty well - and then they just stay there. And many slowly get worse over time due to not training hard or often enough. They teach but rarely train. It is very common. If they do well, they maintain what they have, though they may improve in other directions - like being a good teacher, or stretching, or whatever. If they didn't have high dan grade status (being in with the in crowd) to keep them up there they would have likely been ignored long ago. Students often can't spot this until they have been around awhile.
It is very hard to spot or admit in the self. Heck, might even be me!