Certain types of batteries have a tendency to hold a "Memory Charge." 

If you consistently use the battery for 1 hour and then recharge it, then soon it will only be capable of handling a 1 hour charge, even though it may have a 3 hour capacity charge. A good example is the battery for airsoft guns that doesn't hold a charge as long as it used to.

With rechargeable batteries and airsoft gun batteries, it is important to periodically drain the battery all the way down, and then recharge it fully. That way it is not used to only consuming a certain amount before being charged. Draining a rechargeable battery greatly extends its life and keeps its usability time more consistent.

Generally, for 8.4V 1500 mAH battery, it will last 900 to 1200 shots per charge. It will depend on how you use it. It should last between 25 and 40 cycles (discharge, recharge).

There could be several reasons causing the issue. Most of batteries only charge for 4 hours, you could have overcharged your battery. Moreover, how long before your battle did you charge the battery, because after about 2 days they lose charge quickly.