DWC or not just look at the girls and you can see the growing tips are quite yellow, or lime green, this is a good indication that they are looking for more food, when it happens in the cycle can also give you a clue of what the issue is, the beginning of flower is where you see heavy use by the plant. DWC usually does not require high amounts of nutrients, sorry for the coco thing I got side tracked, if your running at 500-550 then maybe try to bump it up by a 100ppm at a time until they green back up. I run AN 3 part nutrient line and I run equal parts through out the flower cycle. Good Luck