We had a male that had this problem and it was corrected by injecting his rats with vitamins every other week for a couple of months. We used Nekton-rep. Just a SMALL amount mixed with water and then injected into his rat right before feeding.
We had tried everything and even spoke with the vet about it. We now use it for all our snakes a couple times a year.
It goes to figure that there could be "holes" in their nutrition, eating the same prey, that is fed the same rodent diet, month after month, year after year. In the wild they would consume a variety of prey that feeds on a variety of things.
Just be careful because it is possible to give too much and do more harm than good.
Kathy