Have you checked whether something is blocking the hose? Not sure what your Dyson looks like, but if it's like mine it will have one or two little plastic flaps at the base of the hose. If something like a small pebble or a button gets stuck in there you may lose power - try having a look there.
Otherwise there may be a build up of fluff in some other part of the vacuum.
Good luck - Jill
Jill, September 2006