Undersized sick plant in a hydro setup

I've got 6 plants in a DWC drip-feed hyrdo setup. 5 of the plants are doing well. One of them is not growing much. It looks like it's suffering from being overwatered, but that shouldn't be possible in hydro, should it?

This plant *is* the youngest of the batch. It sprouted 3 full days after the rest and it didn't go into the hydro tank until 4 days after the rest of them. I peeked under the net pot, and it does have a decent set of roots that are just starting to emerge from the bottom of the pot.

Other details -
Air temp is about 90 (can't seem to get it lower - then again it's 80 outside the closet right now ).
pH is 5.8
Nutes - running Lucas Formula 0-5-10 - currently at 520ppm
Res temp at 80
Lights - 32w CFL directly over the plant.

Any ideas? (I've attached a pic of the largest plant for comparison. There's a HUGE difference between them.)

 

It was, up until today. I built a waterbreak with the clay pebbles just in front of the plant to prevent that. Think that might be the problem?

Yeah, I saw that too. That's why I moved to the Lucas Formula since it was pretty obvious that the feeding schedule provided by GH was NOT designed for MJ. HOWEVER, I didn't drain the rez since it was filled with a fairly weak solution (250ppm), I just topped off the res with nutes mixed according to the formula.

I've just done some math, and I think that was a mistake since it looks like I have less potassium (nice call btw lol) than the Lucas Formula would produce if I'd started out with it. Fortunately I can probably adjust that when I top off the nutes again tomorrow morning.

Alternately, I may just drain the reservoir and start over

P.S. Just looked at some more numbers. I cannot get the ratios correct by just adding nutes. So I guess I'll be draining the reservoir tomorrow. happy happy joy joy lol.