If the motor is humming while the blades are failing to work, this typically means that the drum has a blockage of some kind.
First step is to remove the black cap from the drum, and then twist the big outer nut with your finger and thumb. You should be able to spin the drum freely with no resistance.
If there is some resistance, remove the hopper and clear any blockages from the drum. Spin the drum back and forth using the nut on the motor to ensure that the whole drum is clean of any possible blockage.
Reposition the hopper back in to place and tightly secure the bolt (this is important as the bolt acts as a safety mechanism for the chipper). Also ensure that the correct bolt is used when reassembling.