I have the same problems. With some of them I'm just happy when I go into lockdown to know whether it is a clear or has any potential. I take the clear out at lockdown just to reduce the clutter in the incubator.
Pointers or hints: Don't get your expectations too high on what you need to see. Broody hens do not candle their eggs, candling won't make any hatch that won't hatch anyway. Candling is mainly for your curiosity. It's fun but it's not life or death.
Candle in as dark a place as you can. Use as bright a light as you can. I don't remember how many lumens my flashlight puts out but fresh batteries can make a lot of difference. One of those candlers can help you see in them better.
Know what you are looking for. I sometimes candle at day 7 just to see if I can see any development. Sometimes I can, sometimes I can't. No big deal. At lockdown I don't look for movement or anything like that in the darker eggs. If most of it's dark and the air cell is at the top I'm good. Some people try to monitor air cell size, I don't.
Keep things clean, don't introduce bacteria to the incubator and don't remove the bloom on the eggs. Your hands should be clean and dry, certainly not oily.