I agree with Hound here; the part of the roots you see is the same part exposed to the aeration first- so naturally it should be the healthiest. No matter how powerful your airstone, if you have the Tangle of Roots from Jumanji, it's not penetrating right, and the USEABLE root area is not enough for the plant.

You are getting advice from a woman very well versed in DWC cultivation, from construction to troubleshooting, and IMVHO her word should be taken quite seriously.