Now that I think about it, I kind of had a similar problem last grow. I ended up transplanting about 4 weeks into flower. One plant I noticed starting dying off late in flower so I took her early, pulled up the roots and they hadn't grown an inch since transplant she came right out and left an imprint the size of the old pot, no root growth at all. The other two girls were fine and are re vegging at the moment. Not sure what happened with the one but she definitely was dying fast.