I suppose it all has to do with the type of feather duster, specific water parameters, lots of things we don't even know about, and whether or not you play Barry White songs to them when the lights go down low. (OK... maybe not that last part.)
Anyway, as far as I know, there aren't feather duster "eggs" you can buy. You can buy the big feather dusters as livestock, but I think you'll just have to let your featherdusters do things in their own time.
Now that I think of it, for the first 6-8 months in my tank, it seemed like I was seeing new feather dusters pop up all the time. While mine haven't decreased in population much, they really aren't increasing now at all either. With any closed system, there's an ebb and flow of population until things balance out. Maybe your tank has just reached that balance point.