Leaking shower drain

I have installed a new fiberglas shower with new brass drain. Rubber and fiber washers are installed correctly and the seal around the pipe seems OK. I can't seal between the shower floor and brass drain and it's leaking. I have easy access to it right now below in my garage. Is there another sealant needed? Nut is tightened as much as I dare.... Thanks for your help.

If the top piece of the drain is flanged, then you need to use plumber's putty under the flanged part to get a water-tight seal.

Thanks - I'll take it apart and give that a try. Appreciate your response.