More ideas:

There's some evidence that poop-out can affect any antidepressant, perhaps any med. The dominant theory is "When a med doesn't work, it can work at first due to placebo effect, but then be conditioned out of working".

Have you been taking the exact same form of bupropion? Different brands of the same med can work differently for some people. Also, if you're taking sustained/extended-release pills, this will have affected your experiments.