Could be just a cheapo battery. I had a scooter for about a year, but it had a cheapo battery that could barely hold a charge, and never could start it using the starter button, so had to kick start it every time. Of course, lights would brighten up and all, though when I would use the signal or hit the brakes, the headlight would go dim. Kept a tender on it, and would charge to 100%, but still couldn't get the juice out of it that I needed. Replaced it with a new battery, and all was well.
Not sure if that's your issue, but might be a good place to start.