show us photos of the opened battery pack ...take voltage of each cell and report back
If you have a bms (battery management system) it may tell the charger something is wrong with the cells, normally they are at 2.7 v each, but sometimes, one of them die, and the others are overcharged and the voltage of each cell rise and create an error in the bms
I saw many problems with dewalt flex 20/60v battery, the cells are not even, some of them get near zero volts, and the bms tell the charger to quit charging .. it does not do a good job, i had cells at 4 volts, and others at 2.7v, and a few at zero volts, even tried to boost / kick them, no avail. They have 15 cells grouped in series of 5.
And the flex batteries are very advanced in technology, tons of smt fuses, and small smt ic's, they are dunked in somekind of clear sillicone, impossible to service.