When you're starting new plants in a bubbler you'll want the water level just below the net pots. As the roots extend downward you can lower the water level.

In my experience it always goes slow at first... seems like it takes forever for the roots to hit the water, then BLAM. Huge roots everywhere. Trust me, once you start to feel relief that the roots are sticking out the next time you look you'll have a reservoir full of roots. It just seems to get massive overnight.