Don't know if anyone suggested this yet but buy some physan 20. I had the same problem, couldn't clone to save my life for about a year. I use an ez cloner and the clones would get little gobs of snot like stuff on the stem and the stem would rot. It's some kind of fungus, the clones that actually root seem to be fine after right? The plant can defeat the crap on its own once it's rooted.
Physan 20 is cheap too, about $10 bucks i think, and you only use 1ml per 10 gal. Just put a drop in your cloner after you clean it, run it for an hour, drain it and refill with plain water.
Edit: After I bought the physan i've had virtually 100% success. That was over a year now and the bottle is still over 3/4 full. Please buy some! I tried everything and killed so many clones before someone let me in on it