There are two possible factors - the head itself, and the pivoting brush-housing and soleplate. Both are unique Dyson designs, and are intended to keep the nozzle in constant contact with the floor.
- Does the head itself pivot freely, as you recline it from upright postion, into your normal useage position?
- Does the brush-housing pivot within the head, as it's supposed to? Tip the cleaner, so it's lying on its back, and rock the brush-housing up and down. Does it move freely, or does it put up any resistance?
It sounds like one of these two pivoting sections is jammed or worn. Have you had this machine from new, or is it a second-hand one?
One other thought - the suction is meant to hold the nozzle against the carpet during use. Are you sure suction is going through to the nozzle? Is the internal hose cracked or split at all?