[GpsFrontier]

Like I said you can always adjust the water level so their getting more moisture. Also you may need to increase the amount of air bubbles if it's not sufficient, and/or change or add more air stone locations to reach the positions that are harder to reach (even coverage). Once the roots reach the water they will wick up water into the growing media above it provided you don't have a large gap between the two. It doesn't take much moisture for tiny roots.

Your problem may also be that if your cubes don't hang down below the raft, the hole opening may not be large enough to allow much splashing from the air bubbles to get to the cubes. In which case there are two options. One increase the amount of air bubbles. Second raise the raft above the water level far enough to allow the cubes to extend below it so there is more surface area for the bubbles to get to. But not so far below the cubes contact the water.