White Widow Clones

I have cloned a number of varieties with normally a high success rate. I usually see roots after about 4 or 5 days and no yellowing. At the moment I'm trying to clone some White Widow. The first lot failed to root. I'm on my second lot now and they have been in about 14 days. They are yellow with green tips and no roots. I'm begining wonder if anyone else has had problems getting White Widow to clone or if they need any different treatment to other varieties.

Any Ideas anyone on why I'm struggling to clone White Widow ?

What do you use for clones? I.e iffy pellets, rockwool? . I make a very mild bloom nute solution and soak my plugs with that, I also mist with it. How often do you mist?

I use rockwool and mist several times a day. I just can't understand why I'm having problems when I've had such a high succes rate with other varieties.