I am not sure about your specific model, but I used to repair vacuum cleaners for a living many years ago. In most cases this kind of fault is due to the ratchet wearing. The ratchet has little right angle steps on them. Over time these wear and become rounded. You can often repair them once only by removing the ratchet and with a little junior hacksaw or file, restore the shape of the right angle. Other than that its a case of buying a new ratchet.
To stop this fault happening again, carry the vacuum very low down the handle, close to the base, so the handle is at a 45', that way there will be less weight on the ratchet.
Ken Tait
March 2007