I had similar problem on my 4th and 5th week from seed.
My problem, I later found out, was my ph was way off on a few waterings.... faulty measuring on my part.
The other problem I had was my soil temp in the plant that was showing the most problems was well over 10 degrees colder than my other plants. Had to set one plant on the floor due to height issues and it was cold as hell.
Not sure if it's the same problem with yours but.....
Check ph of the stuff you put in, the runoff that comes out, and the temp of the medium you grow in...
Temp not only affects the ph of soil slightly but the ppm ratio as well.
Temp also affects the plants ability absorb nutrients... Soil below 60 degrees F locks out phosphorus among others.
Like I said, this helped my girls.... Hope it helps yours.... Because the fix is relatively easy... Good luck