I just reached level 40 in cheesemaking and I can only place 2 casks. Theres a bonus you get when you reach level 40 in that you can now place 3 casks to age. I asked in Help chat and someone told me to check if I didnt have one cask placed somewhere but I dont. I am also growing 2 boxes of mushrooms at the same time, if that matters. I always place casks in crystal cavern.