Axolotl doesn't eat anything, what to do?

My axolotl hasn't eaten anything for two weeks. When I try to feed him, he keeps looking for the food but doesn't bite. He's already an adult, and I've always had problems feeding him. His digestion seems to be fine, as he defecates regularly and undisturbed (when feeding goes well for a change). The axolotl isn't known to have swallowed too much gravel, and he also looks relatively emaciated. The temperature and water parameters such as nitrite, nitrate, and oxygen are all fine. What should I do?

For your information: I feed them once a week, including either 4-6 axolotl pellets or 4-5 pieces of zander fillet. My other axolotls cope well with this amount and eat well.