Keiser when cloning any plant, you want to make sure the cutting reflects the most growth-heavy phase of development. This is a general rule and for Cannabis it may be bent one way or the other, and I don't pretend to know the rules in depth.. The experienced peeps here like catbuds are the best resource. But in general a plant in flowering stage is not putting chemical energy into things like rooting and vegetative growth, so it's hormone balance is all wrong for cloning. Take tops that are actively growing and healthy and you will rarely have problems. Unless your screwing up the rest of it LOL. I have made 15 clones of my Cannabis and every single one has thrived. I just followed the advice of my elders to the letter. I can hint you to directions that have worked perfectly for me (I can't post links) but there are some good guides right here in Cannabis.com. Gotta dig!