A few years ago I bought a clown loach that was signifficantly bigger (well 2 inches most but looked larger) than the group I had. He was a return by someone to the fish shop and most probably had been kept alone for years. He had a hard time integrating into my small group of 5. For the first year I rarely saw him dancing with the rest. He'd in fact wait for them to go to bed and then come out and do flips on his own. He also claimed a separate cave and at least 2 if not 3 years slept alone although there was plenty of space in the hollow tree the rest stayed in. When I moved them to another tank, I put his cave right in front the front glass to tank to discourage him to go in there. He still found it and he moved it himself towards the back, he was that stubborn at sleeping alone. Roll forward 4 years, he's now a very well integrated member of the group, sleeps in a large pvc pipe with the rest. He is the largest and the most outgoing loach of the lot, handfeeding and the likes. However, the only reason he joined the rest is because he can't fit into his old bed anymore, not lenght wise or girth wise
Stubborn loach
It seems to me larger loaches may not integrate that fast, even though mine had no peer and fought with no one. And even the new baby loaches I bought in a couple of batches took a couple of weeks to stop straying from the established school. They initially keep to their own and its not love from first sight.
As far as food goes, clown loaches are able to withstand very long periods of starvation. There was a study done on baby clowns on that matter..., but if a fish is not eating for a month, there's something more than depression going on. It could be worms, it could be hard time getting used to your water. An excessive amount of water changes may perk up a sick fish. Also, the initial squabbles with your other loaches may have triggered some parasites/worms to take over so it is a good idea to deworm as smjack suggested.