What I have read, says to disable all the shutdowns and
let the PC deplete the battery to zero, where it turns off
because it must, and then charge it up. But seems like
you've been down that road already.
You may have a pack with one bad cell which runs down
somewhat, quick and then hangs on with the rest of the
cells delivering but the BMS "thinks" it's deeply discharged
when only one cell is.
Up to peeling off the plastic and measuring individual cell
voltages? Probably not going back together neatly....