Since it seems that all other components have been replaced or verified, it might be worthwhile to pay particular attention to the pump and the air outlet. You could try replacing or examining the rollers and tubes within the pumps as suggested by MEP001 and check if the air outlet is obstructed...
Interesting discussion on vacuum cleaning methods, folks. I agree with the idea that pleated filters can provide better surface area for filtering. It's been my experience that they can be easier to clean and replace as well. As for the vibrating tool to clean out bags, that's an intriguing...