I had 3-4 plants do that in this last little cycle, so I just cut some more clones and to be honest the new clones are already bigger than the ones that are a month older. I feel your pain brother. I would say it's soil because I believe that's the problem I had, but I was also using PH down, and the water where I live is actually almost PH neutral, so I severely fucked up the PH.
What is your PH at? Sorry if you already said this in another post, I have a bad memory
. Umm, I'd say transplant into a slightly bigger pot with some more soil and water with PH'd water. All you can really hope for at this point to be honest.
Also, they WILL green up and liven back up if that's all they needed (transplant and better PH), but if it's not that then I have absolutely no idea. Those are very, very small for 7 weeks. I have plants that have been vegging 2 weeks that are bigger than that (I'm not running my mouth at all, like I said, I was having the same problem but worse over here I think), I can compare the ones I have (that are at least 6 weeks old in veg) to the ones that are 2 weeks old and can see that this is not how it's supposed to be.
Live and learn though.