They are remarkable well able to hide themselves, and if you have wood, they will cling to the underside so tightly, even if you take it right out of the water, you simply might not see them.
I find mine do show themselves, provided they have a decent current to feed in and somewhere to perch so they're in the current most of the time. Either the filter output or a separate circulation pump can provide the current. These guys prefer to filter feed. If you see them picking food off the bottom, they are hungry and not getting enough in the filter feed method. You can try adding Golden Pearls in the 5-50 micron size, green water, bakers yeast dissolved in water with a pinch of sugar and let stand for ten minutes before using, or flakes ground to the finest possible powder to help supplement their diet.
So they get the most chance to get these foods, I turn off the filter for a half hour, but leave the pump running, to push the food around the tank enough times they get most of it. Then put the filter back on.
You don't have to shut the filter down, but it does help them get all the food they can from the water column.
If you get a few more, I find that numbers help them feel more confident and they'll show up more often. Mine spend most of the day sitting on a sponge tied to the side wall, feeding in the pump current I have aimed at it.