Merry Christmas Gillian,
There are only two possible reasons you're having this problem. Mechanical, in that the power brushes are broken, or not enough suction. If you can rotate the power brushes easily by hand and as you said there is no hair or other debris clogging the mechanism, then the problem must be lack of suction. The filter at the top of the motor head could be the problem. Take it out and soak it overnight in water. In the morning shake it underwater vigorously. You might be surprised at how dirty the water gets. I used to clean mine regularly by rinsing it under the tap, drying it and replacing it. I found out that is not enough. Good luck