Al is on holiday so I will take the liberty of answering while I am babysitting his grow (but not the thread!). We both grow the same way (well mostly). Both Al & I get 100% nearly every time in 40mm rw cubes. We both use heat mats, Al has his clones in a thin plastic tray on the mat, I don't bother with the tray. What temp is your mat set to? Should be 30C. Is it a horticultural heat mat or are you using something else like a heating pad for people? Heating pads get too hot even on low. I tried to use one and it always got too hot.

You're watering your cubes too much. See the 1st page of this thread for info on how to water rw cubes. Dip only a corner of a rw cube in watering solution for a second or so.

Overwatering will cause slow rooting, yellow leaves, floppy leaves etc.