Usually that's a pH problem, but it depends on when you measured it. One very common problem I've seen is that people tend to over press their cheeses. This stops it from draining properly. Excess lactose gets trapped in the middle. Since it takes time for the salt to penetrate into the cheese, the starter cultures just keep acidifying the center of the cheese. A good way to tell if this is the problem is if the outside rind has good texture and then about 1 or 2 cm in it goes crumbly. It may also be a bit damp on the inside, or have some holes.