As a natural - presuming you have been lifting hard for many years, at a certain point you reach a genetic ceiling. No matter how much you eat, no matter how heavy you lift, you won't put any more muscle on. Its over.
The key is to realize when or near to when you reach that point and adjust training priorities.
I remember getting fat strong in mid thirties and would be pushing a lot of weight. However, once I dieted off all the fat, I didnt gain any more muscle since I had already reached my genetic potential prior to that bulking period. It was a waste of time and only put my in danger of risking injuries (thankfully didnt happen)