Some castings are harder than others to achieve a good seal.
Some 'plumbers' are klutzier than others.
Care and attention to detail can usually solve the first problem.

If you mean the place with the large nut and rubber ring, that typically does not get teflon tape. If anything, teflon paste (Rectorseal 5) can first be smeared on the threads prior to tightening (that's what I do). I would loosen the nut fully, lower the rubber ring, smear the threads with the stuff I mentioned, then retighten.

Homebound, Yes, that's exactly the place that the water is leaking from! And you are correct, there's no tape there, it's lower where the threads are...
I will tell the plumber about the Rectorseal 5.
BTW, he said something to the effect of "it takes a while for those plastic fittings to seat, so leaks are normal"
I think he's full of sh!t
Thanks for the info!

Generally, the drain is in two (or more) parts. The part that goes in from above (silver or whatever) uses plumber's putty to seal it. You put a small 'rope' of putty shaped like a doughnut underneath this part, between it and the sink, around the hole. You then press the part down into the sink by hand and wipe away the excess.

From below, you place a rubber washer, usually conical or pyramidal in shape, around the drain tube and press it upwards against the bottom of the sink. The cone-shape faces upwards, the flat side down. This is pressed and held into place by a large flat washer and a hand-tightened nut. Tightening compresses the rubber washer against the bottom of the sink and the edges of the drain tube. It also further compresses the putty mentioned above.

Under this you usually have the tailpiece that goes into the trap.

Since all of this is at zero-pressure, you don't need teflon tape or anything else. Just a good snug fit. The seal comes from the plumbers putty from above and the rubber from below.

weedmeister,
that all makes sense but when he came back to fix leaks, he broke sink...too much tightening, I guess.
However, what do you think would make the other sink leak then stop for about 2 weeks then start again? Nothing has changed...
he said he used plumbers putty and teflon tape...
I am confused & hope he can fix it.

One thing I've done wrong before is to put the conical rubber washer in upside down.

One of the most important components in the entire assembly is almost always discarded as trash.

Properly there should be a plumbers putty or silicone under the top flange where it meets the drain opening.

The tailpiece is then passed down through the sink and their is a rubber cone washer that goes up against the bottom of the sink. ( I usually put a light amount of silicone on the cone washer where it contacts the sink

The next piece that should be put on, and the one that is nearly always discarded as trash is a thin cardboard washer that is the same size as the top of the metal locking nut.

Then you put the metal nut on and hand tighten it. After its hand tightened you should give it about 1 to 2 full turns with a wrench.

The cardboard washer is an anti friction washer so you can feel the force required to tighten the gasket rather that the slip friction between the nut and the rubber gasket.

There should be no teflon tape on anything

... and hand tighten it. After its hand tightened you should give it about 1 to 2 full turns with a wrench. ..."

This may be way too much tightening.