Only reasons I can think of:
A slow draw on the battery that is more than the recharging capability of the stator (extra lights, etc).
Loose or corroded battery cables/connectors with too much resistance to allow recharging of the battery at the full rate.
Poor stator or rectifier performance, not putting out enough power.
Battery has been drained too low too many times and wonít hold a full charge. Probably not you issue since it has good power to start when cold.
94mm bbk, +4 stroker, Hot Cams, Vertex piston, FMF Power Bomb header and Power Core can, KW black diamond valves/dual springs, FCR39, K&N, Elka long-travel LT fronts and rear, Houser +3, +3/4 LT A-arms, American Star pro xs chromoly tie rods, Dyna non-programable, Tusk +3 axle.