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.

You have a runt pheno. Nothing can be done about it and almost every hybrid strain will have one. I pluck 'em out. If female, there's nothing that says it won't produce good quality smoke, though. I had one once that I finished and only got 13 grams from it.

The runt pheno would actually be ideal for anybody that wants to do a PC grow.

You could always have fun with it and make it a mother plant, practice low stress training on it, and just overall learn what works and what doesn't. For all you know you could turn that runt into a beautiful mother with the right care.