The "depression is lack of serotonin" theories seem to have less and less base every day, but SSRIs don't only act on serotonin (just review their side effects).
I've done, mostly, vortioxetine and sertraline. For some of us, some SSRIs or SNRIs work. Some don't. Probably it's not because of the serotonin. They have weirdly different effects depending on the person. But there's something going on that we still don't understand. Both because we don't understand the whole mechanism of depression, nor WTF the SSRIs and SNRIs are doing in our mind and body.
At least in my humble opinion.