Your clones are not getting enough oxygen around the stem in the cubes. IMO cubes, plugs, soil all suck compared to aero cloning. I have not used it much but have been having 100% success rate with the one I made for about $20. When I started and tried jiffy plugs and soil I didn't have much luck, some but not much. And I have not used any extra heat. But I do try to keep them warm. You keep the water line lower than the bottoms of your cuttings so they are suspending in the air not in the water. And the bubbles coming to the surface pop and spray them with fine droplets of water. You don't need an air stone like the one I have, Just a good size one. I use a 30-60 gal. air pump with double outlet I got at Walmart. But they didn't have any stones that I liked so I went to big pet store for a better selection. I'll post some pics but let me know if you have any questions. Last batch rooted and were ready to plant in about 16 days. Started showing roots at 7 days and at day 10 all 6 were showing roots. IMO it's the only way to go. All you have to do is lift the net pot out to see how they are doing and then just put it back. No guessing about if they are starting or not. Pic #21 is 12 days from taking the cuttings. Pic #20 is after they started to root enough to take the top off. I might work faster if I had a heater in it but this is good enough for me. I just keep the whole thing at about 80 degrees.