30 liters would be roughly 7 gallons, I can't Imagen that being a problem for plants that size even though peppers are heavy feeders. If the plants that are in potting soil are getting the same lighting and are growing faster I would think that the lighting is sufficient. I would half to say the problem must be in the
nutrients. I don't know what you are using because you said it is a special blend so I cant even look it up. I would consider talking to the guy at the Hydro store to see if he has any thoughts. Also from the pictures and your statement that there are 5 emitters I am wondering if they are being fully saturated. From the pictures, it doesn't look like is the baskets are getting the spray but I am sure that is because you have the lid up, although you say the pressure is lacking. I know that with areoponics it is important to fully saturate the root system and most people overlook this because with each emitter they loose pressure. With one row of emitters down the center the outside of the baskets wont be getting the solution. If I understand correctly you are also using a
drip system that should offset that problem.
I guess, through the possess of elimination I would want to look at the nutrients. You may want to look up the pepper plant pH level, I kept my Bell peppers at 6.0 but yours might want something different if they are different type. At this point, because of your problem I would want to change the nutrients each week until I was able to learn more (just in case). That amount of
nutrient solution for those small plants you could cut back to about 4 gallons of solution if cost is a problem, and if you have that ability? Also more frequent nutrient solution changes cuts down on algae and bacteria problems having the time to grow. I had a similar problem years ago with a single strawberry plant, I had growing in
grow rocks. I never checked the pH because I didn't have a tester, but I don't think that it grew at all in a month that I gave it a shot. I was using dry mixable nutrients (I don't remember what kind). If the system is not light proof then algae and bacteria will be a problem.
I am looking for pumps that will support something like 24 to 48 emitters minimum.