Silica will help beef up the stems and help with that flop.
I think it's the medium... Rbdf just said he grows in soil. Flowerfarmer uses straight coco. Idk that blumats are well suited to a chunky/aerated coco mix.
This is in the 5g pots right? Before giving up on them, I would try growing the same size plants with those blumats, but try two plants in 2g, and 2 plants in 3g.
I have found that the smart pots with coco work better in much smaller size pots than you'd think. It has been said that the max pot size is 5g with the reg blumats.. but maybe that doesn't apply to smart pots, or highly aerated substrates. I don't know for sure, but what I do know is I moved from 5 to 3 to 2 and now prefer growing in 1g's with coco. I don't know which would work best for you with the blumats but I'd say its worth experimenting.
i would try 1 more time.i only used them in soil but kept getting better results every time.if you get 20% of the wet weight,that is almost 500 grams.i used a version of the ppk and liked the blumats better.i also would hand water 1 or 2 times a week since i thought the blumats were barely keeping up with the three gallon pots.either way best of luck!