If you only checked it using your voltwatch, did you make sure it was on the 6V setting rather than the 4.8V setting. If it was on 4.8V and you checked it good at the beginning, you were probably on the back side of the charge curve and most of the battery was probably discharged before you started. Was it a Nimh that had been sitting for a while before you used it? They have a pretty large self-discharge (unless it was an eneloop or similar).
Just some food for thought.
Good Luck,
Curtis