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Well, there's a chance if you put a 'new' one that was a bad thread or badly manufactured, that could be it rather than operator error.
My go-to is the 'vacuum bleed thingy' off Amazon, the one with a handle you squeeze having attached to the open bleed nipple. It sucks the fluid through and you just keep topping up until you see no more bubbled. Then, close the nipple, squeeze the lever a million times, and leave zip-tied closed for a while. This worked every time especially on tough to bleed rears.
Have you tried replacing the bleed nipple? Maybe you’ve chewed the threads.