Check if the motor spins correctly when the belt is off it. If the motor spins ok, then the belt may be worn and need replacing.
As it gets older, the friction increases and the motor finds it more difficult to turn the belt.
You could try adding some belt lubrication as a short-term remedy on a worn belt:
Does the motor spin correctly when the belt is off it?
Ok, let me see if the other experts have any idea on this.