I do not know how helpful this experience will be, but I will relate it anyway. I had some tree frogs that had come from tadpoles a friend of mine rescued from a drying puddle. At certain times it seemed that they would completely ignore their food, crickets, for up to a month at a time. They never seemed unhealthy and they were always very active and loud and reproduced with no problems. The only thing I could attribute it to was changes in the seasons. Also, we have to remember that frogs/ turtles are cold blooded. They really don't need to eat quite like us mammals do.
Also, a friend of mine had a turtle, who eventually dug out of his yard and escaped, but he had a similar experience as you have. Basically, he fed the turtle table scraps, and it became to lazy to ever bother with any feeder fish. They all survived and are now rather large goldfish in his pond.