A lot of balls go off the feed during the winter. If this is a recent problem, it could entirely be due to the time of year. I had one particularly finicky female who'd refuse to eat for as many as 6 months running some years.

You might try insulating the tank with one of those metallic emergency blankets from walmart. You can cut them up and tape them around the bottom & sides of the tank. I once bought some velcro and set it up so I could stick the insulation on at night and take it off again during the day. That could be enough to bring the ambient temperature up.

What are you feeding it? Frozen thawed? Rats or mice? Is it male or female, how old? How long hasn't it been eating properly?