According to what I was taught: nutes, temperature, lights and pH are what you have to keep in balance. Verify those four issues. I don't agree with Stella420 because because I've noticed a definite droop when flowering begins due to the fact that they're expending all their energy for reproduction and mine do look kind of puny. If you changed the light schedule and nutes at the same time they have good reason to droop. If you've ever been on a diet you'll understand that. But I'd keep an eye on the pH as well.
I'm going to suggest something an instructor mentioned as a better method than what we're all doing right now. I plan to try it on my next grow: Start with 20 hours of lighting and reduce it by one hour a week. By the time you get to twelve on/twelve off all you have to change is the nutes and it's not going to be as big of a shock to the plants. If you think about it, in nature the light change is more gradual than the way we do it. He may even have some ideas for changing the nutes. I'll ask him. Anybody want to comment on that?