what it could/probably is, is a bad battery. I've had this happen with mine, if one of the batteries gets too low to a certain point that it won't charge and hence won't light up the charger light. try putting a portable booster on the battery that won't light up. This should "kick start" it and from there you're light should turn on. Thats the only time I've ever had a light go out on mine. I'm pretty sure your charger is okay as long as the other lights are on. another possibility is that there is a loose connection but I'm sure you've checked for that. As far as leaving them on, "they" say its okay to do that but I for one don't like it as charging batteries tends to create alot of heat. A solution to this is to plug your bank charger into a timer and have it kick on once a day for 2 or 3 hours and then kick off through the timer. This creates in essence a trickle charge and will help prevent damage to your batteries and/or a possible fire and at the same time keep a decent charge in them. Hope this helps let us know when and how you get it resolved.