A good place to start would be to thoroughly clean the battery terminal posts and cable clamps. You may also do a voltage measurement on the cables when disconnected from the offending battery to make certain that you are getting power to the cable clamps. Once certain that you are getting power to the battery......check the battery. The battery may be bad and “not” accepting a charge! If the battery is bad, and the pair are a year or more old.....I recommend changing “both” batteries. With both batteries the same age, you get a better charge balance with the batteries! memtb