Hope u haven't cut yet......since u have failed attempts I suggest and a lot of people do and dnt but give it a nice shot of nitrogen and a shot of bloom before u cut .....a clone can only survive on what nutrients it has this is why they turn yellow most of times nutrient depletion by giving it a good dose of nutes will help the clone not yellow and the p in the bloom will help with rooting
my cut goes like this
feed bloom and grow and prozyme Heavy 2 waterings before I cut
i cut my clones few nodes longer than I need trim it up to what I like and make final cut dip in gel and into medium
spray all of the clones with water put dome on dnt take dome of for first 2or3 days take of and spray put dome back on
day 5 roots open the dome vent and hardened off transplant into dirt foliar feed tea
I get 100 % of the clones to root no stunting of growth and always green
just just rambling on hope this helps try it one time