Leaks! I'm desperate please help!!!!

I've been a remodeling contractor for over 20 years. I just installed 7 delta bath faucets and the drains are leaking on nearly all of them. I can't stop the leaks! I'm going insane and broke at the same time. The leak is occuring at the drain where the drain meets the sink. A trickle of water can be seen coming through the threads. They are metal drain bodies. I must have always had the cheap plastic in the other 19 years doing this. Btw I have been gooping the shit out of the connection on top with silicone. Over and over. With the same result. Leaks and more leaks.