Not an expert, but from my experience I have had episodes when I'm on medication which has resulted in a dose increase (not setraline). I'm not sure if I had side effects with a dose increase or if it was because I felt lousy anyway it was really hard to tell.

Personally I don't think my medication had stopped working, I just think that everything that was happening at the time was too much for me to cope with so the symptoms out weighed the benefit that the AD was having.