600W led for your first grow? What setup?
LED setups are notorious for delayed flowering response (and less flower development). The new designs are taking this into consideration and compensate for it.
The explanation for this is very technical and has to do with phytochromes. With led setups, %Pfr (percent of total phytochromes in "active" state) can get much higher than with HPS setups. When the lights turn off, %Pfr slowly goes down till it reaches 0, then its internal "darkness timer" starts.
When you shine light on the plants at night, %Pfr shoots back up from 0, and wakes it up.
Since HPS lamps can't get %Pfr as high, it takes less time to fall to zero after lights out.
Sorry if this makes absolutely no sense. Bottom line is LED's are tricky to make flowering work right. Vegging is always great though.