This is my experience so please take this as a grain of salt, it may vary from others. I have the same type of frog carnwellie. He was an eating machine for the first 6 months (got him as a baby) and then slowly started eating less and less until l he stopped eating completely. I tried everything to get him to eat (warm baths, clean enclosure, turn up heat, different feeders ect ect) nothing worked. He started doing this around December so I though maybe the season change was the reason and he would start eating again by spring summer.

During those winter months, I was assist feeding him every 2 - 3 weeks so he would eat something. Its up to you if you want to do this or not. I did this because I thought he would never eat again on his own. Then come spring, he started staying on the surface instead of being underground all the time and started eating again on his own. So keep that in mind it could be seasonal?

Others say change the substrate once a month, I usually go 2 - 3 months between changes with no ill effects. I do however mix/churn the substrate every few weeks to mix it all together again, as they usually sit in one area and it would be contaminated and hardly touch the rest of the enclosure. Again its up to you as to how often to change, but changing it sooner is always better than leaving it.

If your frog is starting to lose weight, you may have to assist feed him. But this is not for everyone and if you are worried about hurting him then dont attempt it.