DIAGNOSIS: The vacuum's internal computer can sometimes get its signals crossed, especially concerning the battery management. A 'soft reset' can clear these minor glitches without erasing any important data.
FIX: Here's the full reset sequence. 1) Turn the vacuum off. 2) Remove the battery pack from the main body. 3) Hold down the power button on the vacuum for 30 seconds. This drains any leftover charge in the circuitry. 4) While the vacuum is still 'off', plug the charging dock directly into a wall outlet. 5) Place the battery on the dock and let it charge, uninterrupted, for at least 4 hours. 6) Reattach the battery to the vacuum and try it.
IF THAT DIDN'T WORK: If it's still not holding a charge, the problem is likely physical, not electronic. The battery has likely reached the end of its lifespan and needs replacing. These lithium-ion packs have a finite number of charge cycles.
HEADS UP: For battery health, try not to constantly top it off. It's better to run it down to one bar and then give it a full charge, rather than plugging it in for 20 minutes every day after a quick clean.