Maximum level of heroes

pasca-loup

Hi everyone!
Some of my heroes are in level 31.
When I use them for a battle and I win, I do not earn some experience for training anymore. It is written that they are in level max. But I do not understand why, because for these heroes I have still some skills to improve.
Do you have the same problem, or is it normal that the heroes can not be increase their level more than the 31?
Thanks for your answer!