Nitrogen toxicity.
Feeding too much or too often (or both).
In soil it is plain water/plain water/feed/plain water/plain water/feed, so that your plants are only being fed every 10-14 days.
If they are auto flowering plants, half strength nutrients are usually plenty strong enough.
At this stage, give just plain water, in normal amounts and at normal intervals for the next 2-3 weeks or until they really pick up, then when you do resume feeding, half strength nutrients only, even if they are photoperiod plants.
Remember, it is very easy to fix a slight under feeding, but very hard to rescue a burned and over fed plant.
No need to flush with large amounts of water, this will just release even more nitrogen from the soil and compound your problems.
Hope this helps, Organoman.