I feel like this is one of those times when I take my dog to the Vet and say look how sick he is! The Vet stares at the dog and says "I don't see anything." And I say, his tail is droopy, he doesn't feel well.
I don't see anything...
I do have some concerns about the rocks on the bottom of the pot though. The soil could be staying wetter because of those rocks. I know its counter intuitive. Check the soil moisture with a bamboo skewer. Poke it into the pot all the way to the bottom. Leave it for a minute and pull it ou. Look and feel for moisture. You will be able to tell if the soil is too wet and where.
PS: Mammillaria love sun. Near a window is too far away.
