Bathtub Drain leakage - recurring failures
We had a problem recently with a leaky bathtub drain. The leak was due to a crack in the plastic drain shoe. The drain shoe extends horizontally from the overflow pipe to the drain in the tub. The drain flange (the part with the stopper or grate) screws into it. I'm guessing that I cross-threaded the flange when I screwed it into the shoe, causing the plastic to crack. (I had had removed the flange a few times to clean gunk from the drain). The solution was to replace the shoe (actually, we replaced the entire assembly, including the overflow pipe since it is all in a kit). Fortunately for us, we had easy access from an unfinished garage.

Here is a diagram: