Second Hydro Grow. Anybody know what's wrong with these ladies?

We're on our second hydro grow. The first turned out much better than I had anticipated......(Kick Ass smoke! ) We have 4 - Sour Diesel, 1 - Blue Dream, 1 - Pineapple Express, 1 - Afghan Kush X Skunk and 1 - OG Kush. Two of our Diesels are dying a slow death. I'm posting a few pics in hopes someone might have an idea as to what's going on. Both appear to be rotting, or "softening", or "breaking" at the base of the plants. I can't really explain in words what's happening and the pics might not be helpful. But, it's worth a shot. Any help is appreciated.

PS: There's a blackish ring at the plants' base, where the trouble seems to be.
Thanks!!!!

Looks like rotting to me, too much moisture, air circulation issue perhaps.

stem rot! what system you running? need to keep the water/moisture away from the rapid rooter/plant base.

Thanks for the feedback! We're using an ebb n flo system. We've got 10 plants growing....All looking healthy and happy except the 2 Diesels. I'm assuming it's rotting going on too.