OK relax, take a deep breath and step outside the lead acid box you are trapped in.I am concerned that if I continue to use the mower without fully charging all the cells I am eventually going to ruin the batteries.
Any thoughts on what I can do to correct this situation.
Last thing you never ever want to do is fully charge Lithium batteries. Unlike like lead acid that must be kept fully charge to minimize stress. Lithium is exactly opposite where they are stressed being fully charged. You will double the cycle life if you charge to 90%, and never go below 10%. This is called Partial State of Charge (SOC) and is where lithium Ion batteries perform best.
As for the one or two cells you are having problems with is telling you those cells have lost capacity and at end of life.
BMS system cannot balance an unbalanced pack. they can only maintain balance with very small differences in capacity.
As for Bottom Balance goes is exact opposite of what a BMS does called Top Balance. A BMS is used to fully charge batteries every time and is the last thing you want to do. Commercial EV manufactures do not allow the consumer to fully charge their batteries. It is the only way they can offer warranties. If they allowed the user to fully charge every time, they would go bankrupt with warranty claims.
OK Bottom Balance does exactly what it sounds like. You discharge the cells fully one time only when they arrive. You wire all the cells in parallel and walk away for a few hours for the cells to equalize. Makes no difference if you Top, Middle or Bottom Balance. If you Bottom Balance, after cells have had time equalize, you then discharge the cells fully until your final resting cell voltage is 2.5 volts. Once completed, install them in series and charge to roughly 3.4 volts per cell, about 90%.
In a Top Balanced system cell voltage is only equal at 100% SOC, but not capacity because no two cells have the same capacity. Bottom Balance on the other hand capacity is equal equal from charge up to fully discharge. When fully discharge all the cells = 2.5 volts. Example if you use 100 AH cells, the weakest may be 95 AH and strongest maybe 105 AH. The pack capacity is limited to the weakest cell. If you Bottom Balance to say 90% of the weakest cell of 95 AH capacity, charged up every cell in the pack will have 86 AH with 76 AH usable if you limit discharge to 10% SOC.