First I'd check battery connections... clean and dry and no corrosion. Next, battery water level. If they are good, then ask yourself if there is some battery drain, such as a light. No drain, and connections and water are good, then it sounds to me like the battery cannot keep and hold a charge. Get one of those cheap battery hydrometers (floating balls in a tube.) You might see that one cell is significantly lower than the others. If so, that's the first sign of the end of a battery.... if so, start shopping!