Batteries don't charge. I have a new battery, and neither of the old or new batteries work in the charger. I get flashing lights only, sometimes red, sometimes green, but in the end, the battery isn't charged

Dust getting in the charger is a problem, Try blowing it out with a moisture-free dust remover, found at most any store. Green flashing means improper power, due to dust or power from the outlet, try a differant wall outlet. Red flashing means the charger or battery is to hot and must cool down before it will charge. The charger will not charge a battery if the lights are flashing. Solid Red light means it is charging and solid green light means it is charged. Plug the charger in first before putting the battery on the charger. If the charger has been droped or hit the floor the coil inside could have broke loose and dammaged the charger requiring it to be replaced.

If batteries are fully drained, I found the chargers will stay in "evaluate" mode, and never attempt to charge the batteries.
I was having this problem, so I rigged s couple wires from a battery that was fully charged, and connected them to the battery that the charger would not charge. I let it sit for approx. 10 min. Then put it back on the charger , and it started charging normally....
It just needed a bit of a jump start..