I've installed a new American Standard round sink & new faucets. There is a leak under the sink at the drain opening. I've tightened the drain as far as it could go & it still leaked. I thought maybe it was because the drain plug and the nut were plastic, so I went to the store and purchased a metal commercial grade lavatory strainer. It still leaks.
Any suggestions?

Starting from scratch.

Make a 1/4 inch ribbon of plumber putty and lay around opening. Insert pipe, add rubber washer and locking nut. Tighten nut 1/4 turn more then hand tight.

Should be good to go.

Chuck

Sounds like Creahands nailed it.
The drain seal's ruptured and leaking. Take the drain apart and do the putty or Silicon Jel seal again. (see image). Good luck, Tom