Dirt Devil Vacuum Issues?

Why is my Dirt Devil Vacuum Cleaner blowing out air instead of sucking it up? I've checked the belt, and it's wearing out, but not broken. I've checked the brush, and removed all debris. I've checked the hoses and cleared clogs. I've cleaned out all debris, but it's still blowing out air instead of sucking up! Dirt Devil, model 086420, made in 99. My dad gave it to me, and I don't have a manual. Can't find anything online....