I don't have the specifics for your model, so I'd just use normal logic, and a bunch of questions...
Air goes in, air has to come out. Is there a strong air flow coming out of the exhaust port of the vac? Could be a leak in one of the pipes, lowering the suction at the hose.
Is there a secondary filter (probably foam) after the bag to keep fine dust (or broken filter bag contents) from getting out and spraying everywhere? It could be plugged.
Is there more than one wall connector in the house? Are they all the same, or is just one of them weak?
Is there a switch on the vac unit so you can turn it on without having a hose plugged in? How's the suction at the wall that way?
Have you sucked up any towels, carpets, small animals, or other things that may have gotten stuck inside the hose, with dirt and hair getting wrapped around them over time and slowly plugging the hose itself?
MasterBlaster, November 2006
