Lead acid batteries are pretty delicate things really, apologies if its been typed above but a battery that is healthy should be sitting at between 12.5 to 12.8volts resting without any load on it, any less than this and its not fully charged/damaged.
There are a couple of ways in which lead acid batteries can become damaged/lose efficiency 1. is overcharging, its quite difficult to overcharge a lead acid on most modern chargers but it can be done, try to put too much charge on for too long and it will dry the distilled water/suplhuric acid mix that sits between the plates up, since all bike batteries are sealed for life if this happens your pretty screwed and its new battery time, the plates can even become buckled and short if this happens.
The second and predominately more common thing is the battery is not fully charged regularly enough and sulphate crystals build up on the lead plates sort of like rust on iron etc this prevents the battery from holding as much charge and can make it seem as though the battery is dead, you can 'revive a battery' if you trickle charge it for long enough or if your charger has a reconditioning mode (from my understanding it just pulses high and low voltage through the plates breaking up the crystals.
The only real way to stop this happening is to keep the battery topped up, I charge mine every two days on a conditioner and even though the battery is old its still in great nick 8-)
Im guessing you probably havent charged yours up regularly so if your lucky a good long slow charge (oo err) should revive the battery (especially if its new)
oops sorry for writing so much :P
