DIAGNOSIS The battery's internal management system is misreading cell voltage due to an unbalanced cell group. It thinks it's full when it's not.
FIX Fully drain the battery. Use the tool until it completely stops. Then place it on the charger for a full 60-minute charge cycle without interruption. This can recalibrate the cells.
IF THAT DIDN'T WORK One or more of the 3-cell groups in the pack has significantly lower capacity. The pack is worn out and needs replacement. Lithium-ion batteries have a finite lifespan.
HEADS UP Do not leave a depleted battery on the shelf. Store lithium-ion batteries at a 40-60% charge state. Long-term storage fully drained ruins them.