I tried to breed with another person's pet and it wouldn't let me. If it reaches the max mucles to breed, then can it be up for breed? I don't know what in the world happened.

xMarker, please check if both of future parents have enough energy to breed and if both of them haven't reached the breeding limit. If this is not the issue, please tell us the IDs of both of these pets and the approximate time when you tried to breed them.