I find my plants droop in the middle of the day. Early mornings they are fine. Around 15 hours they droop and stay that way till next morning. I run 20/4 light schedule.
It might be light stress, depending on how powerful your lights are 20/4 is a long day cycle you could lower the power of the light or raise it higher and see how they respond to it.
It's normally to droop a bit after that times, but it should be good when the light turn off. I got mine on 18/6 and most of them drooped at 14-16h. But can't tell you anything without pictures.