That looks like salt build up. With fabrics pots you need to clean them with hot water between grows. I used to get the same thing when starting out. Clean everything between grows.
Always start auto's in final pots. Yes, they can cope with re-potting but not ideal. It looks like your leaping from one issue to another, and not able to solve problems because of over-compilation.
Next grow start with the basics and nothing else. Simplify and organzine grow space and grow methods. Only then will you know what aspect has thrown a plant off. It's difficult for us to provide a definitive answer, as we don't know all the steps taken.
I'd do some split testing to find the issues...
Repot 1 in to clean pot with fresh soil and don't overwater.
Stop feeding another.
Feed more nutrients to one.
You may have to sacrifice a few to keep the others.