I was having issues cloning as well. And after a couple tries, I dished out the dough ($150). I know it's a lot of money for something that should be so simple but it was an impulse buy, I had the money....so fuck it. I also bought a ph meter and found the water I was using had way too high of a ph. I started cloning my plants as well around weekd 5 (plants were almost 3 feet too) in jiffy cubes. I then bough the cloner, took the ones I had in jiffy cubes for a week, and 3 of those 5 rooted. I also took a bunch more clones and 15 out of 18 had 1 inch roots in about a week+1 day. I bought the 24 spot one cause I'm going for a SOG. Only problem was, my big plants are about 4 more weeks away from finishing and I didn't have another light. So I bought 4 40w fluoro's. Hopefully, this will be enough until I can get them under the MH/HPS.
If you getting frustrated with cloning like I was and have the $, get the same set up I had and it will work. Or you can do what other's have suggested (I've heard it's easy with the pad warmer, jiffy cubes/rockwool and cover but it didn't work for me).
Good luck man. Let us know how you do.