[A] Why I have to change/wash my filters more frequently than normal?  
It is possible that your bin is not properly sitting in its place.
The cone seal is not properly placed at the top of bin inner or it is not covering the rim of bin inner.
 Filters (except lifetime post-motor) must be changed or washed regularly as recommended in your user guide. Dyson filter
change intervals are based on normal domestic use. Any non-domestic use may drop the filter performance.
Dyson does not recommend using carpet or bag fresheners as they can adversely affect filter life.
  
[B]  Why my Dyson switches off?  
All Dyson models have a thermal cut-out device for safety. So, if anything causes your cleaner to overheat, it will cut out.
It will not restart until it has cooled down and there is no reset button. Leave it for at least one hour. If the room temperature you are vacuuming is very high the cut-out may be triggered spontaneously.  
However the most likely cause is that your filters either need changing or washing (check your user guide to see if your filters are washable or have to be replaced), or check your cleaner for blockage. Don't use the cleaner again until you have ensured the filters are clean and that there are no blockages.

[C] Why brush bar stops turning?
 If you have a model with brush control (DC03, DC04) first check that the brush bar is not turned off. If the control is set to N (neutral) the brush bar will not turn. The control should be set to  P (park) position; it will then automatically go into D (drive position) when you push down the head of the cleaner to start cleaning.
Check that there is no blockage preventing the brush bar from turning. Refer to your user guide for instructions on how to clear blockages.
If the brush bar is still not turning, it is possible that the belt driving the bar has broken. If you own a DC03 or DC04 please call Dyson helpline 
0870 527 5104.
If you own a DC01 you can replace the belt yourself and spares are on sale from our website. Refer to your user guide for instructions on how to replace the belt.
   
[D] Why dust ceased spinning?   
Don't worry, your cleaner is probably fine! As the bin fills with dust, the dust can settle and cease to spin.
If no dust at all appears to be getting into the bin you may have a blockage or need to change your filters.
See your user guide for instructions on how to change your filters and clear blockage.

[E] Why my Dyson performance is poor?
Check that your filters are clean, as they should be either washed or replaced regularly to maintain optimum performance (check your user guide to see if your filters are washable or have to be replaced).     If replacing or washing your filters does not restore the performance of your cleaner, there could be a blockage in your cleaner.    Please refer to your user guide for instructions on how to change your filters and clear blockages.  
All DC01s, DC03s, DC04s and DC05 Motor- head models have a rotating brush bar in the head of the cleaner that helps them pick up dust and pet hairs: if this is not turning, when the cleaner head is down read [C] above.