Plant going limp and I don't know why.

Hey.
I opened the cab this morning and I noticed that two of my plants are going limp. The sunleaves and tips to the branches are limp as if it's approaching death. When I pick up the dixie cup that they are in there not dried out. These plants have been in the cups for 1 1/2 weeks and I always water them when the cups are dried out and very light; but these cups had nutes in them still. What could cause the plant to go limp like that if it's not dried out? The ppm of the solution has been kept between 350-400 and they are in the 2nd week of flower. There are other plants in this cab with the exact same setup and they are fine. Anyone had this happen to them and fixed the problem without loosing a plant that's been growing fine for 2 1/2 weeks?

They may be root bound,repot them,flush them and then let them dry out a bit and then feed them with half strength nutes and see what happens

Yeah, I'm not sure what else it could be. I didn't know that too small of a container could kill the plant. When reading the bibles it says it stunts growth, but I think telling the reader that it will kill their plant they'll make sure it doesn't happen.

Thank You very much sir