The other problem that I can think of, if it has an aftermarket motor is the tube that runs from the motor opening to the bag chamber not being installed properly. I guess a gasket issue with the original motor would also do it.
I suppose an electrical problem could also cause these symptoms, if the motor is running slower than it should.
The other thing that I can think of is the 1205 simply not being as powerful as later models like the Super J. If your comparing it to a modern cleaner, you will probably be disapointed.