that was my 1st guess... less light. distance from the light makes a big difference too. i think i read that 18 inches is optimal.
EVERYTHING makes a difference, colder temps could slow growth i bet and if they're underwatered, they won't grow as much. plenty of nutes will help too.
a tip i picked up that might help too is to rough your gals up a little by twisting their stems. that causes damage and when the plant repairs it, you'll not only get strongerstems, but better nutrient uptake. that's why it's always recommended to have an oscillating fan... same thing, just not as drastic as bending & twisting.
even putting beans in a warm humidity dome will speed up their early growth enough to give them a head start. i'm not sure, but i think a lot of my beans last year weren't doing well because my room was too cold. it affected strains from a bunch of breeders at different banks even, so i know it wasn't just a case of bad beans and in fact the C99s that were 100% just a few months early were something like 3 out of 5 when i was having trouble with EVERYTHING.