Dyson vacuum will spin fine in the air, but stops when it hits carpet. ?

I have a Dyson canister vacuum. I don't know the model, but it's red and gray. The roller brush spins just fine in the air, but stops spinning as soon as it hits the carpet. So it's not sucking up anything on carpet. I've cleaned the brush of all hair and lint. This has worked in the past but not now. I've tried clicking the turbine control button while it's on, off, and in various numbers. No dice. Any suggestions.