Leaking drain (pop-up) assembly

I installed a new drain/pop-up assembly in a new bathroom sink and I can't get it to stop leaking. It seems to be leaking between the Mack washer and threaded section of the pop-up body. I have tried tightening the locknut in half-turn increments to no avail. Is it possible that I have over tightened it causing a distortion of the Mack washer and the leak? I didn't have this problem the two previous times that I installed a similar assembly.

-Steve

Drop the washer and nut down a bit, and push some plumbers putty into the threads. Just a little bit. You don't want any putty on the rubber washer, just in the threads.

Without the putty, water will follow the threads down, past the rubber seal.

Thanks Terry!
I read the instructions, and they do say something about putting plumber's putty on the threads, but they are not very clear. I thought they were referring to the threads where the tail-piece attaches, not the threads under the washer.

Regards,
Steve

I don't think what I posted will be found on any instructions.
You asked why it leaked, and it's a common question.

It's the answer that any plumber that installs faucets would be able to tell you.

I have no idea why the people that write the instructions, never have to install the things and run water through them.
To us plumbers, it's a no brainer.
At least after we've fixed a few that way.