i had a very similar situation in which one cell was lagging all the rest. i didn't want to believe it could be a bad cell, but there was no easy way to access the cell in order to troubleshoot and test. The cell "seemed" to be working in that it would charge and discharge, so maybe if it had a little help to get up to the level of the rest, it would be okay?
In the end i was lucky to still be under the 10-year battery warranty, so i let the dealer have it and they swapped out the pack. If you are under the battery pack warranty period, then you should get a new pack from Mits.
If i had no warranty then it seemed to be 2 options:
1. open the pack and remove the lagging cell, then test it on the bench to determine capacity, self-discharge, impedance, etc. If it checks okay, then charge up the lagging cell to the level of the rest and re-install.
2. open the pack and replace the cell with either a new one or a used one from a salvage pack that had been tested on the bench.
If you just open the pack and charge the one cell without testing it, then it is a lot of work to repeat if it turns out the cell really could not be saved.
Did you see this thread? viewtopic.php?p=23601#p23605
i have to think why would a cell be lagging? The only reasons would be some extra load on it from the CMU board (defective CMU), or it just has an internal defect. If it has a self discharge, then it is defective (internally shorted), and it can't be repaired by charging.
Keep us posted on what you find out about your cell.