@nemo801 the charger does what's called a top balance. I.e. charges cells to the top of the voltage range. Draining the battery by running it flat in a milk crate or similar does what's called a bottom balance (bottom of the voltage range). Don't do bottom balancing too often but it might help in this case. If the bottom balance doesn't fix it, a cell is dying and you should pursue a replacement battery under warranty.