Yeah, then it sounds like upping your dose may be worthwhile. It's possible you've built up a tolerance to it.

The other thing is, sometimes antidepressants stop working after a while. Basically, they work great for a few months or a few years, then they just randomly stop working one day. I don't think it's common for that to happen, but it can/does happen for some people. So, that's something to consider.

The medical name for it is "Tachyphylaxis".