A bit of droop at the end of the day is normal but I have noticed that mine are more prone to evening droop when they are becoming rootbound, but I'm growing in soil. Coco can tolerate a lot more root-binding and those pots look like a decent size, so it's unlikely that's the problem. Maybe they need more water, coco is hard to over-water anyway.

Have a look on youtube at some of those time-lapse videos of growing, plants do often droop towards the end of the day and I don't think it means much more than they are "tired" from the day's hard work in the sun. Here is good demonstration of that: YouTube - Cannabis Time Lapse - Marijuana Grow #2