My potato has large lumps in it and after baking and mashing it the lumps are not going away. What is this?

Those of us old enough to remember mom’s wide, hand-mashers will recall that even after a thorough mashing, enough unmashed lumps remained and was indeed characteristic of the times. I suggest to ensure an even consistency in the mashed potatoes, that you use a good mixer or ricer. Take care not to over-mix however, or the starch cells will break down too much and the result will be a gluey-like texture.

All Things Mashed Potatoes HERE