A lot has to do with where you took it from on the plant, how long before taking the clone did you feed the mother. Obviously some strains are different in regards to how they clone, but most times it has to do with the things I mentioned rather than the strain. As I said, some are different but not much. Some are harder but the more I use the ez-cloner which is basically fail/idiot/anything -proof, aside from lack of electricity to keep it running. I think it makes cloning way way simpler than any other method. I know everyone is gonna say yeah I get 100% etc... yeah so do I and the only thing I have to do is not kick the plug out aside from taking the cut and whatever else mentioned here. Not that I'm lazy in any way at all but I'm a firm believer in human error, murphy's law or whatever you wanna call it, but rooters can dry out on you, dirt is slow, rock wool... as a organic dirt grower do you really want that shit on your plant going in your dirt? I can live without it. Not to mention you always gotta keep buying that stuff. Every once in awhile you have to replace the 2" pucks for like $15 per 50 or something.
Back to the regularly scheduled response...some factors will cause rooting to take longer... Nothing wrong with it/them but it just takes a bit longer, its usually when you start seeing the leaves look like shit/showing deficiency's when the roots will start popping for you; I.E. no more food left in the leaves so they toss out the roots to find go some.
Next time you take clones take it from the bottom of the plant if you can, if you can avoid giving them N or any food for that matter as long as possible before taking the cuts that will make it shorter. Otherwise... you see what happens, they just take forever, I've had them go over a month. I think you can get cloning times down to the 5-7 day ready for dirt if you really tried tweaking and proper timing of taking cuts from the mothers.
I dont really worry about it myself but I just give myself enough time, if you are running on a time frame, then it may be worth starving them a little if possible, and trying to keep the lower branches looking healthy, I know thats what causes me to take cuts from the upper part of the plant, I veg under flouro's so I dont get enough light penetration to my lowers which end up always looking like shit...
Whats in your cloner as far as a solution?
I use straight out of the tap water, no bubbling or RO, its not worth wasting on this. I run it as warm as I can get it (not hot, 70-75, some say higher but I really dont worry about it aside from keeping it above 72 or so) and in order to not have to worry about a nasty slimy cloner and water, as I fill it up with warm tap water, I throw a couple oz's of hydrogen peroxide in... I'll start with 2 or 3 oz's in my 60 site ez-cloner and fill the reservoir up until the misters are just in the water, as much as possible without hindering the sprayers/misters... And I never worry about PH either before you ask, I run the same water with no rooting hormones, no nothing aside from the peroxide and as of recently, after 4-5 or so days I'll throw a tablespoon or so (thats all I've added so far) of general organics ca/mg+ and a couple will explode with roots after adding the ca/mg.... I dont know why they do that, but it does work for whatever reason... I'll have to look into it but I tried it for the first time on my most recent batch of clones.... a couple from 1 plant shot roots quickly and within 2 days they had enough root to take them if I needed to but I like to get a nice set of roots going before putting them in dirt, but no roots were there prior to putting in the ca/mg+.
I honestly cant say enough good about this method of cloning, there is no easier/better way I feel. Seriously, I've done it every which way and this is the best I think. You dont need the $400 ez cloner either if you'd like to try it, look for DIY bucket cloners that use a small pond pump with misters etc.. The mister frame can be made out of 1/2 or 3/4" PVC and fittings.