Thanks Jojoe.
I'm starting to think it's just plain old overwatering. Looking more closely at the plants, the leaves are "fleshier" than I'm used to, if that makes sense, and the purple markings have faded somewhat. I'm going to let them really dry out between waterings, and see if that makes a difference. I had been letting the pot go pretty dry (and the ones in my old office were dry most of the time), but I think I subconsciously picked up the pace when my Hoya flowered for the first time last year... now it looks better than ever and they're suffering, so obviously they weren't meant to share a pot. I need to get over my "succulents are succulents" mindset and figure out their individual needs
I want to say these plants are about two years old, but they could be three. I've only had the Hoya for four years, and let it get established before I used them to fill in the gaps in its pot. I've had Kalanchoes (clones of these plants) produce after just a few months and for at least six years, so I'd lean toward conditions rather than age being the culprit.
It sounds like you're in a similar climate to me... when you put your plants out for the summer, do you need to keep them under some kind of cover to avoid the rain overwatering them for you? I've been scared to expose my Kalanchoes to the dangers of the world after losing my oldest and biggest plant, but it sounds like most people do it successfully, so maybe I'll give it a go this summer.
