one of my plants is staying small

Well, halfway through my veg. One plant is staying so compact stacking leaves.this its my first time indoors. my geuss would be some kind of root problem. I have researched but lack exp.

Sometimes that happens when growing from seed you mite get one that just wants to grow retarded keep the best one as a mother and you won't have to worry about it anymore

Few things could be the issue:

low nutrient strength or nute lockup.. low medium temperature, overwatering, phosphorus and/or calcium deficiency.. How are your conditions? What's the temp near plants with lights on and off? pH of the soil of the plant having the issue? temperature of the medium? what does the moisture meter say for soil condition? If everything else looks good, I'd point to the genetics of the plant.