Maybe you should hook the battery to a trickle charger, make sure it’s fully charged, and get a Voltage reading with a multimeter.
Then start the engine, and while the engine is running take a voltage reading at the battery with the Multimeter.
Compared to the initial Voltage reading, you should get a higher Voltage reading while the engine is running. If the voltage reading is the same, or you get a lower voltage during the 2nd test, the problem is somewhere at the charging system.
If the Voltage reading is higher during the 2nd test, you should get a proper load test done on the battery at a battery dealer, or you can do it yourself if you have the right equipment.
If the load test proves that the battery is fine, something might be draining battery power while the scooter is not used.