So when the Dyson works on the attachment hose, but doesn't work when cleaning the floors, this can be caused by a few things.
The first place you should look is at the bottom plate. Sometimes if the bottom plate is taken off to clean the brushes, it is very easy to install it back on the vacuum the wrong way. Here's pictures of both the wrong way, and the right way to install this plate. What happens is that the bottom plate is actually higher then the brushes, so when cleaning on carpet the brushes don't actually hit the carpet, decreasing the pick up on floors dramatically.
Here's a picture
of the WRONG way to install the bottom plate. Notice the gap in between the two housings:
Here's another picture
, that is the RIGHT way to install the soleplate:
I would say probably 6 times out of 10 this is the problem with the customers machine. If you checked the soleplate and it is installed correctly, move on to the second possible problem.
If you check this part and it is correctly installed on the machine, here's something else that is pretty frequent on Dyson's that would cause the same exact issue you're having:
Possible Problem # 2. Turn the vacuum 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 is in the reclined position. When the first few series of Dyson DC07 models came out, they didn't make this hose long enough, so when the vacuum 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:
This would also explain why the vacuum works fine when using the attachments, but not on the floors. The attachment hose uses a different suction channel, and when the machine is in the upright position, no suction goes through this hose. 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
I know that you said you have checked the clog ports for obstructions and there weren't any, but I'll just paste the picture of both clog ports on the Dyson just in case:
Other things you may want to look for is the length of the actual bristles on the roller brush. The Dyson vacuums use a very cheap, thin bristle on their brushes. The bristles wear down quickly, and need be replaced I would say every 2-3 years, maybe even sooner if you have pets in your home, and vacuum a lot. Here's a picture of the Dyson brush (this is the DC07 model, not sure if this is the model you have) when it's new so that you may compare it.
If none of these suggestions fixed your vacuum, I'd be more then happy to walk you through some more advanced diagnosing. Please just add a comment using the Clarification request form here on Fixya, and let me know which exact model you Dyson have, and if you found anything new while trying these fixes above.
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