If it's always cradle bound: Your battery has probably died an early death or it needs to be seriously recharged (8hrs +) and conditioned. Lithium Ion batteries need to be used / conditioned or they lose their ability to hold a charge very quickly.

My personal rule with these types of batteries: On receipt of a new L-Ion battery or device using a L-Ion; fully charge it and then run it into the ground and re-charge.

When in regular use, don't keep it on the charger all the time or it won't hold a charge. L-Ion batteries hate being partially charged; this actually reduces battery life.

PS: This is general experience with this type of battery. Philips does recommend that you keep the little guy in the charger stand when not in use; thus guide yourself accordingly.