Like you said, Root Riot are sponges. ^^
And yours are wet. The root may don't get enough oxygen and drowned a bit.
You should check if the PPFD (the amount of light your lamp emits) is over 400μmol/m².s, I suspect too little photosynthesis.
With coco, you will need a little cal/mag added in. Coco doesn't hold it very well. What ever ec of cal/mag you add, decrease the grow fertilizer by that amount. Lots of nitrogen in most cal/mag suppliers.
This plant should have been planted long ago. Pellets don't hold what the plant needs. Get your planter ready, adjust the ph for coco to 5.8 and plant her with regular feedings. You can find a feed schedule on my profile page for soiless grows.