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Hello @Kev1-Towns
From what you have described it would appear the batteries are either reaching the end of their service life OR they have experienced charging issues which have had a compound effect over time.
Smart chargers are great but it has been my experience that they are designed to not accept battery packs whose voltage has dropped below their designed "minimal acceptable" overall resting voltage, the battery management systems (BMS) integrated into the packs help to prevent over charging / discharging but a battery should NEVER be left in a low state of charge because whilst the discharge is lower for this type of chemistry (Lithium) is still occurs and once you reach that cut off voltage the Smart Charger will no longer work on that pack.
If still in the Warranty period, I would go that route 👍
Ozito 18v PXC Battery/Charger User Manual
"But I always thought Lithium doesn't deplete itself to death (from overuse)?" - Noyade
Actually @Noyade lithium cells do have a limited number of cycles in their lifetime (300 - 500), as they age the capacity also reduces thus older well used batteries have a lower overall capacity and will reach an unserviceable voltage sooner than newer battery packs if left in a low state of charge, temperature also has an impact on battery cycle/health (further details in link below) 👍
