Have you checked (with a magnifying glass) for spider mites and their little webs between branches and trunk? Do you fertilize? Have you checked how rootbound it might be? What kind of soil mix is it in?

Yeah, there are definitely no mites or any pests at all. The soil mix is a pretty shite mix of pebbles and potting soil, with what looks like some small mulch, too. I've been meaning to repot it, but I don't think winter is necessarily the best time to do so. As far as fertilizing goes, no not really. I've heard that diluted milk does the trick, though, so I might try that when I track down my spray bottle.

What? Diluted milk? Give it some proper house plant fertilizer, anything balanced (e.g. 15-15-15),