Nirvana Big Bud grow

Wot's up all,

I have 10 Nirvana BB growing at the end of week 2 now. I woke up this morning to find slight brown patches on two of the plants only on the first set of leaves on two plants. I have noticed that the leaves are twisting and not uniform in shape. The other 8 are fine and even the dodgy 2 look pretty healthy. I have a 400 hps, good air flow/circulation, heat is stable at 76 deg with a 9 deg heat drop in the dark period, they are currently on 18/6. Havent started using nutes yet ( except a very mild mix 1/4 strength when very first girminated ) but I have done that before and dont believe that is the problem but that said I must say it does look like nute burn, how can that be, please tell me im wrong. The other 8 plants look the healthiest I have ever grown at this stage.
Any suggestions guys/gals?

Since this picture was taken about 8 hrs ago the brown patches on the nearest leaf have increased 50% in size

Still looking for feedback people, the problem is now apparent on 4 other plants. All comments greatly appreciated.

could be nute burn,but i think the p.h.is wrong and no nutes dude untill week 3-4 nutes in early life premotes males