I had a similar issue when I first switched from HPS to LED. It wound up being less of an issue of light intensity and more about my VPD needs being completely different under LED than what had worked perfectly for me for years under HPS.
I would make sure that your environment and VPD are ideal first before tweaking any other parameters of the grow like moving the light, changing feed etc. The jump from 500 to 900 ppfd could be causing the issue, but I had the same issue with drooping plants under 500ppfd, and bringing the Temps and Humidity up (around 80F, 70rh) brought things around quite quickly after tearing my hair out for weeks thinking it was overfeeding/underfeeding, light burn, ph or root issues. My low humidity was causing transpiration issues that led to a calcium toxicity which brought on Magnesium and Potassium deficiency, and the first signs of problem was always the drooping.
All that said, 900 is a lot of light for that stage of growth, I would have that at about 4-500 max, get the VPD in range and watch them take off