electric battery powered mini moto ?

these are just over 200 new..been used a handfull of times then it started playing up when there is no weight on it the motor turns the wheels fine...once there weight n it the motor struggles to turn the wheel... could only be the battery not putting enough charge to the motor...or the motor either way its a easy fix.. the wheels can be seen turning and the motor working...c/w charger... 07513 719 922 for a faster reply....