How often do you drive it? If you dont keep the battery charged it can quickly lose its ability to recharge. If its jusr ditting in the garage you probably need a battery tender yo keep it charged.

as a battery discharges a growth called sulfation occurs on the positive plates. When you recharge the battery much of the sulfation is removed in the reaction and the specific gravity of the electrolyte will return to normal. If the battery is left sitting uncharged the sulfation continues and hardens, preventing the chemical reaction from happening, and sometimes growing enough to penetrate one of the plate seperators causing an internal short