I struggled with this issue since 2009. Two years ago, I figured out EXACTLY what it is and how to fix it. I have clone onlys as old as 1986 and OGs as old as 1991 and I can get them to root in rockwool in 5 days. Before, it would take 21-25 days to root them. People used to try and clown me for talking about this issue, like the leaves going yellow in week 1 even after a heavy dose of Nitrogen. After figuring out the cause, they are asking me for advise lol. Im not about to give up years of work and have all my credit stolen. Its pretty sad but I will let the community figure it out, if they ever do.