When the Cyclone Assembly on the Dyson is not spinning around, it usually means that you are loosing suction somewhere. This can either be caused by a clog, suction leak, or dirty filters. I will give you detailed step by step instructions along with pictures on how you can diagnose this.
Possible Problem # 1. Turn the vacuum cleaner upside down so the brush roller is facing upwards. I found the easiest way to work on it is to put it on a counter top or work bench. On the left hand side behind the roller brush you should see a flexible transparent hose that goes from the motor to the agitator housing. This hose actually has a lot of strain on it when the vacuum cleaner is in the vacuuming position with the handle down. When the first few series of Dyson DC07 models came out, they didn't make this hose long enough, so when the handle was in the reclined position, it would put too much stress on the hose, and cause the hose to actually crack, usually right where the end of the hose that goes into the agitator housing. Examine your hose and see if you have any visible cracks in this hose. If you need to you may just remove the entire hose assembly by pulling on it. It will easily come right out of the machine so you can inspect it better. Here's a picture of where the hose usually cracks:
If you do find a crack in this hose, the this hose needs to be replaced. Here's a direct link to that hose on our GoVacuum website.
Dyson DC-07 & DC-14 Upright Replacement Internal Hose Part # 904219
Possible Problem #3. Dyson's in general are prone to clogging. There are a few clog ports throughout the vacuum cleaner that were added there due to Dyson prototypes becoming clogged at these points. Here are pictures of how and where to check for these clogs. Simply remove the suction port covers at both of these locations:
hope this helps you .........