Have all the Transformice steam achievements except Immortal Shaman. It requires 10 000 saved mice as shaman and I already have 11 808. I will be asking on the steam forums too but I thought I might ask here too. Why is that??!!

achievements are often bugged, there's really nothing you can do about it unless you want to cheat it

Like the player above said, can have happened one glitch on game.