Well you could add some mg and see if it helps or hurts. I think it's bugs. They're not on your plants 24 hours a day....especially white flies which come and go. You say you are seeing one thing but the symptoms are showing another. You have to go by what "the evidence" is as they say in the forensics world.....just because you personally haven't seen bugs doesn't mean they aren't there.

Here's a guide so you can look through different symptoms and perhaps it will help you decide.
International Cannagraphic Magazine Forums - The Complete guide to Sick Plants,pH, and Pest troubles!