Hi KaTheya,
I apologize that I don't have experience with your problem. I've often read members discuss that if their tortoise is kept too cold this reduces their appetite, but I can't speak from my own experience.
If you don't get some good answers here, you may want to try posting again in the Russian Tortoise section.
Good luck!
What are you offering?
What are your four temps? Warm side, cool side, basking area and overnight low?
What equipment are you using to maintain those temps?
UV? What type of bulb?
How often do you soak?
What substrate is it on?
Im offering spring mix and the grassland tortoise food sold at the pet store. I have the solar glo 125 watt uvb bulb. I have one side of the tank on 95 for the day 70ish at night and the cool side is around 70. The substrate is the loose coco fiber mixed with some type of wood stuff I got at the pet store I dont remember what it was called. I did just recently get that bulb and I think it has been helping because I have seen him eat alittle more I also sprinkled timothy hay around the tank and yesterday I saw him eating a bite of that.