Stunted growth

Okay, so I think/HOPE I have the crumbling leaf problem under control - but one of my plants is substantially more stunted than the other. I got them as clones at the same time. What is going on with the little one? Is there some way I can encourage it to grow more or should I just give it time? For the first couple of weeks I had the two of them, the little one didn't even show any growth in the middle - those little leaves in the center of the plant are a very new development.

I've uploaded images from both plants for comparison's sake. They're growing in soil, under a light for 18/6.

Any ideas? Thanks!

Bump? At this point there is additional growth in the center of the plant, i.e. more leaves sprouting out, but there is *still* no upward growth - so it's getting more and more crowded in the center, and the leaves are staying very small. What's going on? The other plant is doing fine, and I got them at the same time and have treated them the same way.