Clown loaches suddenly nowhere to be seen

Hi - I'm a VERY amateur aquariast. My wife & I bought a house with a
tank built into the wall, and the past six months have been a crash
course. We've got two angels, two clown loaches, a couple black
mollys, a host of tetras, and an algea-eater I've only seen a handful
of times.

The clown loaches are by far my favorite fish in the tank. When we
first moved in, we didn't see much of them - that was for like the
first couple of weeks only. Then they were always out, chasing each
other around, exploring, etc. Always front and center.

The other night, my wife pointed out that she hand't seen the loaches
in awhile. As I thought about it, I hadn't either. The past couple of
days I've been watching closely - they don't come out even for
feeding. For awhile, I feared they were dead inside one of their caves
(a large log with openings). But today, I was using a flashlight to
see inside the caves, and I could see one of their tails. It was
moving, so I know that at least one is alive.

Why would their behavior change so drastically and suddenly? We
recently had a high-PH event (ran out of PH- and the store we use was
on back order), but that's long past and thing have been back to
normal for awhile. That's the only factor I can think of that could
have impacted their behavior (all of the other fish seem unaffected -
it only got to about 7.5).

Any help for this blooming newbie?

Matt