First, take a voltmeter and verify that the battery is indeed fully charged (esky chargers are not very good). If you can, it also may be worth checking the voltage under load, while the heli is spooled up. An old battery may be fully charged but not hold the voltage under load.
If the battery is good, the second thing I would suggest is to do do a timing mod on the motor. To do that, gently bend off the little tabs on the top lid of the motor and rotate the lid 5 mm clockwise. There are detailed descriptions and instructions with pictures somewhere on the net.
This mod will give quite a bit of more power to the motor.
Sergey.