Have the same problem with mine. Everyone quit on me. Eating DF and *fresh killed. You could try a live rat and kill it before offering. This works well with most of them. If they still refuse then just cool them down to high of 82, low of 79-80.
If their weight is good, etc. no vet visit necessary. They're just doing what comes naturally.

* Grasp rat's neck with thumb/forefinger, and base of tail with other hand. Give a quick jerk. You'll feel the neck snap. It sounds bad, but drowning or suffocating is worse. Most zoos put the rodents in a box, wrap in strong plastic, and CO2 them.