This is actually a very common problem that we see for repair on Dyson Vacuums. Usually what happens is that the vacuum doesn't loose suction like most people think. You can test this by feeling the suction power on the hose. If you have suction coming from here, then most likely the machine isn't clogged.
What happens is that the bottom soleplate on the vacuum cleaner can easily be re-installed the incorrect way. If this happens the brushes on the vacuum cleaner can not even hit the carpet, which makes the vacuum feel like it's not picking up, but when the suction is tested from the hose it has plenty.
We see a lot of Dysons come in for repair with this problem. The bottom plate is not flush with the rest of the vacuum cleaner. Here's a picture of what an improperly installed bottom soleplate looks like:
When the bottom plate is put back on the vacuum, you should make sure that the bottom plate (the purple colored part in the picture) is flush with the agitator housing tray (red part in the picture). If not, the brushes will not hit the carpeting, and the vacuum won't really pick up anything.
This is how a correctly installed bottom soleplate should look like:
This should be the first thing you should check on your vacuum cleaner as it's the easiest and most common solution. If this doesn't fix your Dyson, please just use the Clarification Request form, and let me know which exact Dyson model you have, and whether the vacuum has suction power from the hose, whether the brushes turn when set to the carpet setting, and how old the vacuum is. I'd be happy to try and further diagnosis the problem with you.
I hope that this was helpful to you. If it helped you, please be sure to rate the repair as helpful by clicking the thumbs up icons in the top right hand corner.
Thank you very much,