I had the same problem when my seedlings were young and I never really got to the bottom of it...the best I could figure is that the plants were just drooping because the hydroton was soaked...I never changed my pump time the 24/7 schedule is still goin and once they got a bit bigger then seemed to perk up more and more so next time I believe I'm gonna let the roots just start to get outta nets before I put them in the tank...if I were you I'd try just turning off the pump and watching them repeatedly...if you notice the plants pick themselves up after a while they are probably over watered...you should be able to tell if they are underwatered because the hydroton would be dry

I'd start with that in the troubleshooting process but like tamsinho said a pic and a bit more info on the room and res specs would help greatly

I'll check back a few times tonight if ya happen to post I'll try to post quickly