I think it's just the size of the tank and the number of opae ula you have. I felt the same way when I had a few in a 1.5 gallon. It just seemed very sparse and empty. I added more and now it doesn't feel empty.
Behavior wise, they mostly just crawl around on the (dead) coral/substrate and graze. There's not much room to swim for them in the 1.5 so I don't notice that behavior much.
In my 10 gallon there's a lot of vertical empty space so I notice them swim and glide around more.
If there are hiding places, a "large" tank, and few shrimp, I think it's just natural that it would seem like they aren't around (and not something you are doing wrong with water parameters or anything).