Based on your information, it sounds as though you may be losing seal on your vacuum, perhaps due to either the operator (?), or uneven flooring (?).
Also, double check your vacuum (air-flow) and make sure there are no obstructions from the wand-to-the-machine vac filters.
That would be my suggestions, but ofcourse, you will solve this situation quickly..
Good Fortune to all;
Ed Valentine
cross-american corp.