Instead of soaking in plain warm water, you add to the water some baby food. Usually plain baby food carrots or squash. You can also use cooked carrots that you blend up. Another thing you can add is Pedialyte, which is sold for use on babies. There is also a Critcal care powder you can add to the water. The thought is this way they may get some nutritional value while doing a soak.
Have you checked your temps, the lights and how long they are on to see if they are the cause? Any other abnormal behavior other then not eating?
Sorry.. She is a RT she is roughly 14 years old. All temps are optimal but I have been having trouble with humidity. She gets a soak every other day. She has a MVB AND A CERAMIC HEATER.