With nearly an inch of engaged thread in the insert and bolt, I don't see how it could be stripped, but did you use a new copper sealing washer?
They are a one shot deal - a new one is required everytime you reinstall the plug to ensure a leak free seal, however, if you dress the old ones down on both sides by rubbing them on a piece of 280-400 paper laid on a flat surface and wetted with paint thinner until you remove the galling marks, they can be reused.
I have about a half dozen and just save the old ones. When I use my last fresh one I rework all the used examples for a fresh supply. Using this technique you can probably get at least three uses out of each copper gasket, and I've never had a leak with one of my "refurbished" gaskets.