I've heard of clowns that just don't grow well. I think they're low on the group hierarchy. Less food, less growth, but maybe there's more to it? I've had slower growers but not any that didn't grow a little bit. It may be hard to compare no growth to slower growth. There may need to be a runt...low man on the totem pole?
I was of the mindset of acting like all loaches may have internal parasites & treating them in QT....after a couple lesser attempts with clowns. It seemed to work until I had a weird clown that would just not eat normally. It seemed focused on anubias flowers & guarded them to the point of not eating...anything else. I put it in QT & treated for pretty much everything to no avail. I finally had to euthanise the poor fish.
I don't think your 1 "not growing" is too terrible, yet. You said it doesn't look emaciated...maybe it just smaller or slow growing...someone has to be most little...