I'd e-mailed their tech support yesterday, and the response today is that it's likely one of two things: clogged filter screens, or maybe more likely, debris caught in the check valve(s). So, since you need new fiber washers (which are a metric size and I couldn't find so need to order as well) in most cases when you take it apart, I'm going to wait for new check valves in case I can't clean the ones there. Frustrating, as the flow (unless I want a cold shower!) is meager, but had to order them.
Next time, when the water has been off, I'll open other faucets first so the check valves don't receive the brunt of the action when the air is purged...it probably wouldn't have been an issue if they were newer, but the o-rings, etc. get stiffer as they age, and are more likely to stick.