I'm using RR plugs and Hydroton in a top feed drip system in my current grow.

Chances are, its another variable. What is your other medium?

Are you putting the seedling in the drip system right away? If so...

How often are you watering?
Does the RR get saturated every time you water?
Does the drip land directly on top of the RR? If so, does it run into the hole at the top?

If not...

how is the humidity in the space?
Are you foliar feeding/spraying directly on the leaves?

Are you using nutes?