I think there are issues you need to adjust and the magic may just happen.

1/ Are your cuttings wanting to grow? This is very important.

2/ Is the light strong enough?

3/ Is the temperature reasonable?

First you need to know if your cuttings want to grow. Plants grow in cycles. They often need a rest then grow again. If you get very healthy growing stems, they might have been getting into a slow down cycle and go to sleep. That's why people often pick stems that have been resting for a while and now start to enter a growing cycle, not the other way around. Put them into the refrigerator for a week may also help for some plants to fake hibernation and getting out of it in your cloning machine's warm temperature.

The light may not be strong enough. You can use energy saving bulbs, coolwhite type and about 20W. I would use 2 of them to supply blinding bright white light to teh cuttings. Beware of the heat though. Let some air flow through to make sure it's cool. A little fan blowing precisely at the bulbs might help to gently remove hot air out.

Temperature should warm at the bottom and cool at the top. The idea is that you should keep teh root zone warmer than the leaves. It's good when you do it in colder temperature and use an aquarium heater to warm the water at the bottom to a constantly warm temperature.

Good luck!