Are you using old seeds? I've experienced similar problems with old seeds (>3 years, and not stored properly)...they sprouted fine but didn't really grow at all, eventually I just tossed 'em.
Oddly enough, I have others of the same age and stored in the same manner, and they worked out fine, so could be strain dependent (or just dependent on how "good" the seeds were in the first place. After recently moving cross-country (my seeds having been shipped in a moving truck in the middle of summer over 7 days, likely scorching heat in the truck), I attempted to germ about twenty beans, various strains that I had around for 1-4 years, and only six sprouted, and NONE of these survived more than a week, so I bought some new beans and started a new grow with fresh seeds. Well, a few weeks ago, just for kicks, I tried germing my last remaining "old" seeds (a five-pack of "Neville's Haze Hybrid" that I bought about 4 years ago), and not only did ALL of them sprout, but they're all doing really well. [I enclosed a pic of them as of today, not sure what I'm gonna do with them now, since I don't really have space to grow them out atm, but I only had five seeds of this strain and I've read that the high from this strain can be kinda trippy, so I wanna keep 'em, and so I just have them under a T12 fluoro light until I can decide what to do, but anyways this strain seems to be pretty vigorous, whereas all the rest of my old seeds went nowhere].
IOW, yes, yours could just be lousy seeds, or maybe too old - or both - rather than something wrong with the way you're treating them. Probly time to buy some new beans. Nirvana seeds are pretty good, and Attitude has a larger selection of strains, you can't go wrong with either, imo. Good luck
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