With this type of charger, a flashing LED would usually indicate a charging fault, usually indicating a potentially damaged battery or one at the end of it's operational lifespan. It could also indicate a mis-match in the charge state of the batteries being used, as the charger will try to charge simultaneously, mixing mostly full and mostly empty batteries can cause charging issues.
If you have one pair charging properly, and one pair flashing, take one of the working batteries and switch it for each of the others in turn. This will help diagnose which battery of the charging pair has the issue.
Date published: 2017-07-24