New DIY Waterfall scrubber just doesnt want to grow, can i get some help please.

So ive made a waterfall ats for my red sea reefer 425,
i have used MakersLed heatsinks & phillips 660nm reds with a mean well driver.
the screen is 6"x6" lit from both sides i have also covered the acrylic box with some kids contact/schoolbook covering etc as it has a reflective silver backing and it helps to keep light mostly in the scrubber - i have 4 leds in each fixture lighting the 6x6 screen.
currently feeding 1 cube of mysis per day not rinsing & 4-5 pinches of pellets give or take throughout the day & id say approx 18-24 square inches of nori daily.

Lighting schedule was 18 on 6 off but i have boosted it to 22 on 2 off in the hope of getting some good initial growth then backing it off to 18/6 etc

stocking is:
blue tang
scopas tang
tomini tang
1 mandarin goby
2 tomato clownfish/pair
3 dispar anthias
foxface

im really wanting to get this scrubber/screen growing well as i want to use this scrubber on my new six foot tank in a few months and add another 2 cube scrubber as my fish start putting on some size as they are mostly around 2" at the moment bar the foxface @ 5.5-6"

its been on for around 5-6 weeks now and i am yet to see any real noticable growth at all
i roughed the screen up quite a lot a few days ago & added some light diffusers to try and get the initial growth kicked in but with the nutrients in + bioload i am seeing no real growth apart from some dark brown on the bottom of the screen/very very light brown which you really have to look for over the whole screen and one strand at the top of the screen where the slot is .. at first it was very light growth barely noticable/yellow after 3-4 weeks i thought photo saturation which is why i roughed the screen up even more due to light growth & added difussers now it seems to have darkened somewhat since the light diffusers were added but i am not getting enough growth to really be able to tell just yet wether this is going to make it kickoff .. i used a scrubber on my previous tank and it grew incredibly fast and i wont run a tank any other way now but i just dont get why this one refuses to grow no matter what i do - i feel im doing everything right to get it to grow but no bananas.

so troubleshooting wise ive:
re-roughed the screen
added light diffusers

not 100% on actual flow but im using a nyos viper 2.0 set to half-3/4 and im getting a nice strong even sheet of flow across the whole screen.
should be well above 35GPH per inch of screen.

any help on how to proceed would be great & much appreciated.