My kuhliis hide very well. I didn't see them for a couple of months and found them only when I moved the tank. I have two, and they disappeared again after a day or two.
I'm not sure if I read it here or somewhere else, but you can make a "cave" of sorts in front of the tank to see species that usually hide during the day. Just lean a small flat rock against the front glass, then cover the front of your "cave" with black construction paper (on the outside of the glass of course) . If you want to be really safe use a plastic resin rock from your lfs
. Anytime you want a peek inside, just move the construction paper.
Another thing I've read about them is that keeping a half dozen or so will make them more active during the day.