2 much Water?

In addition to what Hashy said. When was the photo taken? If it was taken close to lights out, then it’s completely normal for them to droop. It’s been a long day under those lights and they know it’s close to lights out and they’re relaxing now. My plants are always dropping 2-3 hours before lights out. Then morning comes around and she’s standing up a proud again. Now if they’re dropping like that all day then yes, you possibly have a bigger problem.

I just checked out your diary. With them being 4 weeks old your going to need to start introducing nutes with NPK in them. I am growing in 16L fabric pots and I water every 3-4 days but I put about 5L through each pot until I get a bit of runoff. Try next watering putting way more water through until you get a bit off runoff.