Weird term, but "poop out" is exactly what experts call it when antidepressants stop working.
Its an odd phenomenon which we rarely see with other types of medicine, but it seems that about one in five people on medication for depression find that the medicine simply stops working.
If youre on a medication, were doing well, then seem to hit the skids, its not all in your head. There are several reasons some antidepressants just poop out. Sometimes the problem is simply a shift in brain chemistry, often triggered by stress, that causes your symptoms to worsen. Other times, the reason is an underlying medical condition such as anemia or a low thyroid; an interaction with a new medication you may have started; sometimes just no good reason whatsoever.
Bottom line, if you have depression and things were going well only, but now the sun has gone behind dark clouds, talk to your doctor. A change in dose, the medicine, a checkup, a fresh look at your diagnosis, etc. may all be needed.