Louis,
I would eliminate possible wiring, connection and switch problems by connecting directly to battery with appropriate guage wire. If still not loud, there may be an adjustment screw on back but that is for pitch and probably won’t change volume. If all else fails, you can take it apart to see if there is corrosion inside. I hear tell that connecting to 12 volts will work for short use but it doubles the current and will eventually fry the coil. Maybe someone has already done that.

Don’t forget ear protection. That advice is from an old guy with hearing loss.