I'm so frustrated please help! (First grow)

I have no idea what's going on with my plants! I've done everything I've been told by the guy at the grow store here in town. I feed about once a week about a gallon to a gallon and a half of water with about 30% runoff.
I am growing outdoors. Switched to 12/12 last week. Two of my plants are at about week 3 of flower, one is at about 2 and my largest are at about 1 week. (I'm guessing by the flowers).
Each feeding I add:
1 tsp of Cal-Mag per gallon
1/4 tsp of open sesame per gallon
Every other feeding I add:
1 tsp of FF tiger bloom
My pH has been at 6.5 every watering.
The leaves on my two big girls (Fire OG) are yellowing and dying off, this is on both old and new growth. As well as a few other issues that I can see.

Also one of My other girls is yellowing as well but differently, no dying off but a little dropping. .
This is My very first grow and I'm still learning. Please help

Typical nute burn also check the ph in soil also may want to decrease the nutes and add some calmag.hope this helps.
You can tank me later noob

most grow stores can be very confusing to a new grower. they carry plenty of choices, but you have to know how to use them. the guy behind the counter doesn't always know what to do. they just talk like they do.

Looks like Phosphorous burn.