Not seeing your fish when the tank lights are on is a normal phenomenon and can be deflating when you simply want to watch them.
Light is often the enemy of fish in the wild. It exposes them to both daytime and nighttime predators. Hiding behavior is therefore inherent, even in domestically bred species. This is often especially acute with the use of tank lights.
Not having a place of refuge, either from you or tankmates, can be stressful on fish. In fact, many fish keepers often provide little, if any, shelter for their fish, which can be a stressor. This is especially true for tanks that are sparsely populated. You can mitigate this by 1) providing lots of shelter in the form of rock structures, caves, wood, plants, ornaments, and other tank contents and 2) keeping the tank lights off longer, especially if you have ambient light (e.g., sunlight or room lights) in the room(s) where your tank(s) reside.
The photo at the top is my Tanganyikan community tank. It has a tank light, but it’s only on for 1.5 hours a day. The room the tank is in has ambient light for about 9 hours every day. The tank light is not on in the photo. The large fish in the center is my Altolamprologus calvus. He’s shy and normally stays on the bottom of the tank when the tank light is on, either behind the ceramic caves in the photo or inside them. However, when the tank light is off, he ventures up into the water column and will cruise around, as seen here. There are also some Julidochromis regani in this tank, which also follow this behavior pattern.
I have posted before about tank lighting and fish behavior. That is just one post. There are others, and you can search the site for those.
There can be lots of stressors affecting captive fish. Lights are often one of them. Unless you need the tanks lights on for long periods (i.e., if you have plants), try leaving them off and see if you notice normally shy fish swim more openly. Of course, if you have sparsely decorated tanks or tanks with high density of occupants, lights may not affect their behavior much.