Could be too warm for them. I haven't had much luck growing lettuce indoors. It always bolts. I assume it's due to heat stress.
You would think so but all of the lettuce above is perked up.
There could be a number of reasons for your issue.
As someone who grows 100% indoors, I too have run into my share of the "droops". In 100% of my cases, it's been a case of watering issues. With that being said, here is a great little document to help you "read" your plants.
I grow a lot of lettuce and from the looks of it, it would appear to be a watering issue. There doesn't appear to be any discoloration or nutrient burn so, this helps narrow it down. I would start by making adjustments to your watering schedule. Usually, "droop" occurs from under watering. Judging by the droop of your last two rows of plants this is most likely that case. In my case, I have my watering schedule at 3min On / 14min Off. You may need a different schedule depending on your indoor environment. What you're looking for is for your Rockwool to appear to start going dry just as the next watering cycle starts.
Lettuce is a sponge. It sucks up water like a Vampire in a blood bank. With that much growing, I wouldn't doubt that the higher plants are taking more than their share (Greedy bastards).
If you do not have a timer that can do this, here is the timer that I've used since I began growing indoors It's not an expensive hydroponic timer but it is a time that's been reliable since I've begun using it. It is NOT rated for outdoor use but as long as you keep it as far as you can and you don't dunk it into a bucket of water, it will be fine. (Plus, it's affordable)
I hope this gives you a good direction to go.
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My 2-cents... When working with a device that places humans, water and electricity in close proximity (such as a TG), it is in your best interest to use a device that is 100% waterproof to the IP55 standard as an absolute minimum.