We had the same problem with our exact same vacuum and discovered a blockage in one of the pipes on the side of the body, but also that the dial needs to be pushed right on Low to suction at all on carpet. After a year of frustration this was quite a discovery. :)
Holly
October 2008
First off check that none of the pipes are blocked, if they are clear then it's probably the filters that are causing the problem, especially the one in the waste cylinder area.
They can be washed but only a certain number of times before they need to be replaced