You're possibly having these issues with phosphorus because of your sips keeping the soil too moist.
Your temps could be too low.
You could have too much zinc, copper, iron, boron, etc in your soil locking it out.
You could need to be doing more feeds/top dressings. Just add water is never gonna be as good as something you're consistently feeding the little guys.
My money is on the sip not meshing with your mix or its just keeping your soil too moist on top of needing to feed your soil regularly.
Free wheeling and doing your own thing can work, sometimes. Sometimes it doesnt. I'd ditch the sips and follow some proven organic regiment. You're over experimenting imo. Trust me, I've been there and done that.