Plants won't take nutrientd

Hi guys. So I had 4 plants. One of them over the course of 2 days turned purple then yellow and then died completely. The three remaining plants all look nice the bud is looking very sticky smelling nice and looking where it should be at this stage BUT the tops have turned yellow?! So I've been checking the EC of the water each day and they are drinking water fast but not taking the nutrients. So I flushed them with ph water for 1 week and then re introduced nutrients at a lower dose, still not taking and still slowly turning yellow.

I'm growing hydroponically with bubble pots, I have no resovour I o each pot separately. I'm using 3 part genesis formula which I've used before with excellent results and they are under 2 x 600w cool tube lights.

I've attached some pictures

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Pics

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Also I'm too scared to flush them again as surely if they are starving and I flush it's just going to get worse?? How can I get them to take the good stuff with the water?!

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This looks normal. Plants yellow mid to late flower. Its a sign that they are using up their stored nutrients. The bigger leaves will yellow out and then drop off.
Some pic's of mine after a flush. They still grew about a month longer. Just didn't ever want to finish, the trichomes never went amber and the hairs stayed white for the longest time. I'm not saying yours will need to bloom for another month like mine did. How many weeks into flower are they?

There 5 and a half weeks!

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What strain is it?

When I first read you had a problem and I read the tops were yellowing I thought you meant the bud itself not just the leaves. Was about to suggest CalMag because CalMag is supposed to help with bud end rot, but from the pic's I don't see any end rot, just normal yellowing from being mid to late in bloom.

The one that died, was the stem dry inside and hollow?
Do you have any more info on the one that did die?
Are you using RO water?
If they need anything extra I might assume Calcium.
But I don't have enough info to suggest much.

Newb here but at 5.5 weeks should they not still be kinda green, i guess strain has to be considered. Looks to me (cant stress newb enough) like a bit of nitrogen may help.

Imo, at 5.5 weeks we might as well just call it 6 weeks. Some strains only bloom 8 weeks, others only 7. Seems like most bloom 9-10, some a bit longer.
At 5.5 - 6 weeks, I would say that fits in the window of mid to late bloom which is a normal time to see yellowing, even more so for organic amended soil growers, not that the OP is growing with amended soil but just saying. At 5 - 6 weeks into bloom, if you double that then you get 10-12 weeks, so this is defiantly the middle of bloom or later regardless of strain, but depending what the strain is, this could very well be late bloom.

I still want to know the strain, and get some more info about the one plant that did die. Seems kind of random that the one plant would have just decided one day to stop taking nutes and die. If it was maybe lacking calcium, then it could have been hollow inside. But hard to help when we don't have enough info about it.