One of my plants has stopped growing (pics inside)

Hello all. I'm almost 4 weeks into flower and all of my plants are 21 inches+, with the exception of one. He's been 13 inches for weeks now. In fact, I'm not sure if hes grown vertically at all since I started flower. It's also weird because it was my biggest bushiest plant all throughout veg. I switched my lights into 12/12 when all of my plants were about a foot. Anyway, some info on my setup:

Seeds - 4 feminized Nirvana Northern Lights seeds
Light - 400w HPS kept at about 6 inches from tops of plants
Soil - Fox Farms Ocean Forest and Light Warrior mix 3 to 1 ratio
Nutes - Fox Farms 3 pack. Grow Big, Big Bloom and Tiger Bloom. Started nuting at 1/4 strength. Currently nuting at 1/2 strength twice a week with waterings according to schedule.
Water - 16 oz bottled water per plant 3 days a week. Water is PH adjusted.
Containers - 3 gallon plastic pots
Fans - 1 big bedroom fan blowing on plants and light
Temps - about 70 degrees, Humidity - 35%

That's about it. It looks as healthy as the others besides its height. The plants currently sitting atop a bunch of blocks so its level with the others.

Anyone have any idea whats wrong with this guy, if anything? On a side note, I haven't done any trimming or anything of that sort during this grow. Thanks in advance.

If there are more plants i would personally like to see some pics of them all together in one pic. I have been been growing NL from Jack herer seeds for about a year. And your NL look excactly like mine! too a t. That tall thing....no but the small bush yes. It simply looks like the bush got more light. ....Most NL you see look real indica with fat leafs.Nice grow!

If you were growing in Hydro I would say to hit it with some Canna PK 13/14™. I'm sure there would be something similar for growing in "dirt".

the one on the right looks healthy and the one on the left looks like she has some sort of deficiency or lockup. do you feed them at the same time from the same nute mix or do separate mixes? those canoed leafs are a bad sign. the one on the right looks nice and healthy though

Thanks for the responses. They're always fed at the same time from the same mix. I didn't even notice how bad the leaves were canoed. Those pics were taken before a watering, but I just took a peek at my plants and the little guy's leaves still look horrible. The others are looking very well though I think.

I'll have to take a look at some of your grows PBF. As for pics, I have one on hand of all of them together, but its not that great. I'll take some more when I get a chance (not my camera). I think the plant in the back right got burnt a bit by my light. But he's doing fine now.

Any advice on what to do?

If you haven't already i would raise the smaller plant to same level as big plants...lookin nice!

Thanks PBF. I have him sitting on some blocks so he's level with the others.

Not only are the leaves canoeing. But they're about a quarter the size of my other plant's leaves width wise.

Anyone have an idea on what I can do? Maybe flush it? This is my first grow and I'm a bit lost. I honestly wouldn't be that upset if it didn't grow any higher, but I'd hate to lose it completely if it worsens.