Need plant help please (in flower)

Yo guys, had this same problem the last time i grew in soil, didnt used to get it this bad when i did hydro...

Basically got 4 power plants in 4th week of flower which are turning yellow, i'd say 40-50% of the plants are yellow and it's not just the fan leaves at the bottom (as it should be) it's all over the plant from top to bottom and some of the leaves closest to the buds are yellowing aswell now. Thing is ive got a white widow plant in the centre of my grow which i feed with exactly the same mix as the power plants and its fine, green all over and looking super healthy, with the most developed buds.

The buds on the power plants are filling out much slower. Im not sure whether im over or under fertilising, its one of the 2. Now should i just keep watering with plain water to see what happens, or should i start feeding every time i water to see what happens?

At the moment i feed em once a week with full strength solution and bud boost, cannazym, liquid silicon. PH is set at 6.4 every time. Plagron royalty mix soil. 600watt hps. im sure im doing things correctly, jut cant work it out. If it helps, the power plants were starting to yellow a bit in veg aswell, whilst the white widow plant has been lovely and green all the way through.

Any help would be great.....and no i cant upload pics at the moment, sorry......

Plant size and pot size? Thinking rootbound.

or magnesium deficiency? pics help alot!

Plants cant be rootbound as they are in 11litre pots! The actual plants sizes vary between 2 and half and 3 feet tall....

i will try and get some pics up on here in about an hour....

plants run thru their mag quite often, right around that time of flower. i vote Mg

If it is an Mg deficiency then should i start feeding them say twice a week instead of once to rectify the problem?

You can try foliar feeding with a 1/2 teaspoon per quart of water. Then maybe add some calmag to your regimen.

I would give them a normal strength watering with Botanicare Cal-Mag Plus.

white widow needs less nutrients than almost anyother strain and by far it wants more calcium and magnesium than any other so chances are if it was a cal/mag deficiency it should have start developing on ww first and then on anything else , add some more N to the solution if u see that the PP are not close to theyre feed limits ( a super dark green WW is on its upper band of optimal feed , a lil more and it will start burning )