What are you growing them in?
I started some clones in new Rubbermaid tubs last year and none would grow roots then after a few died I moved them to pots of ProMix HP and they did fine.
6 clones in one tub were rooted in screened promix and 3 in another tub were rooted in a DWC type cloner I made. The new tubs are made with a different type of plastic that is touted to remain soft at cold temps and I'm wondering if there is something in the new formulation that inhibits root growth. I've done about 50 grows in the older tubs and never once had a single clone not do well.
Each tub got the same startup nutrients. 1ml/L each of AN 3-part nutes with a dash of CalMag in RO water so they were around 350ppm. After a month still no root growth and the clones were obviously suffering and like I said and some died.
I didn't pre-rinse the tubs but never had before so maybe it's from a chemical coating used to release the plastic from the molds they are made in or something in the plastic itself.