Do you know the kind of thermostat? I'm just wondering if maybe your temps are off. Does the thermometer have a probe that you can measure the hot side with?
If the temps aren't it, it might be a security issue in which case moving her to a tub or a smaller tank might help. Alternatively you could throw a bunch of crumpled up paper in the tank to provide some cover.
I had an adult ball python go on a hunger strike for more than 6 months. Before that, she was only eating sporadically. Since I got her she was a great feeder, it was just recently she decided she wasn't hungry anymore! I brought home a trio of ASF rats and the next day she took a f/t from me. This was about 4 weeks ago.