NFT Hydroponics Question: When do I move seedlings to the main channel?

I’m brand new to hydroponics and just built my first system after watching a lot of videos. I thought I had a good grasp on it, but I might be missing something.

My setup has two sections:

• Bottom section: where I start seedlings in rockwool. The water level is high enough to keep the rockwool moist.
• Top section: where I transplant the plants once they’ve developed a larger root system.

I just checked the top section today, and the roots that aren’t sitting in the water feel pretty dry, and the rockwool cubes are also dry. In the top section, the rockwool does not touch the water. It's about an inch off the bottom. The top of the plants don’t seem like they’re getting enough water dispite water running through the tube and the roots are sitting in it..

My question is: at what point should I be moving plants from the bottom to the top? I thought the roots were long enough, but maybe I jumped the gun.