It may be that your belt is broken, or stretched out. This will cause the brushes to slow down dramatically when vacuuming the carpets. When the belt stretches, this causes the machine not to pick up on the carpets, and makes it seem that the vacuum is loosing suction.
In order to test this, turn the vacuum on it's side while it's on. Are the brushes spinning? If they aren't, this is because your belt is broken. If the brushes are spinning, the belt still may be stretched. Listen to the brushes as you put them onto the carpet. Can you hear a change in the sound like the brushes are slowing down? If you put just a little pressure on the top of the vacuum by stepping on it, do the brushes stop then?
It may also be the filter is clogged in the machine and needs to be replaced. Bagless vacuums use a HEPA filter in the dirt cup instead of a bag. With a bagged vacuum you just replace the bag when it gets full, but on a bagless vacuum the HEPA filter continually gets debris and fine dust accumulating on it, and needs to be cleaned, and replaced occasionally.
Here's a link to the owners manual for your vacuum. This will explain step by step on how to replace the belt, as well as servicing the filter.
If you still need some help, please just use the comment field here on Fixya to ask your question. I'm always more then happy to answer all follow up questions, and I really am not completely satisfied until you are able to fix your vacuum.
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