I think a decent root stim product with humates is very good for seedlings and veg, thats all I use. Builds some fantastic roots. Personally I have got to the stage, if a plant isnt doing the biz I scrap it, especially if I have something to replace with, I know it sounds drastic! mines more to with room, really got fed up with trying to nurse plants back to health, plus electric, time involved and you do get poor phenotypes sometimes, its nature. Took me ages to realise though and was unwilling to do it at first, but really as made my life easier. A lot of photoperiod growers only select the best to flower, and compost the weak ones. I appreciate Auto's are different but I am now applying the same principle. Probably also depends how far you are with it, if its only a couple of weeks left, fair enough, finish her off.