There is supposed to be plenty of potassium from the other two macros if they are dosed. I dose potassium if I decide to use less of one of the others though.
Agree about alternating macros and micros, it is remotely possible the iron and phosphate are precipitating out and doing the plants no good.
This is the suggestion there -
50% water change
barrs general hardness booster
Days 2, 4, 6
Days 3, 5, 7
How much light and what is the lighting period? How is the tank stocked?
Perhaps there isn't enough nitrate and phosphate for the stocking level.
I have had plants perk up after dosing additional magnesium via Epsom salts.