Either there's a blockage in the air path that the hose uses or the hose is split and air is getting in at the split.
You may be able to work out which by listening to the sound of the motor - a blocked hose allows no air to flow and the motor runs a little bit faster because it has no load (air) to move and it therefore makes a higher pitch sound.
Lots of good Dyson repair help at dysonmedic.com
Peccavi, April 2009