Loss of suction on hose Dyson DC04?

I recently took apart my Dyson to replace the belt, and since that time whenever I stand my Dyson up to use the hose, it makes an awful squealing noise and doesn't have good suction, just seems to move the dirt around on the floor. My dad suggested that I had put the Dyson back together wrong after changing the belt, but I've done it so many times before and have checked it and it's fitted correctly. I'm frustrated that I cannot hoover my stairs, please help!