If the drum can turn freely, the motor can supply the pull required, and the belt is properly fixed and tightened, the system should be running properly.
So to check :
When you turn the big driving wheel mounted to the drum, does the motor turns also?
A : If so, the problem is in the motor.
B : If not, the problem is either in the motor itself or in the belt tension.
If it is the belt tension, the spring loaded tension wheel is not properly mounted.
If it is the motor, just take of the belt, and check the motor : is it running free?
Connect the dryer to the mains, and switch on with an empty drum.
Does the motor turn now?
If not : is the motor humming slightly?
If you turn the motor, does it start running?
Do you have a multimeter to check voltages ?
Please reply to the questions here and contact me by private mail (click on my name and follow instruction) , and I will advice you here further.