This can possibly be Phosporous deficiency.
You need to consider the mix you provide, pH and any other info that may be key and not included here.
50-60ppm is usually where you want P. If in soil, you ramp up toward that when you think you are providing 100% of nutes.
oh yeah, soilless, so you provide 100% all the time...
just ensure soem wet-dry cycle and 20% runoff... up the P a bit and it'll get right back in line. (assuming all other things are fine)