I have the A6000. It has power to the suction

belt worn or stretched

It's an issue that the front roller isn't engaged. there's usually a belt connected to the roller on one side of it. That belt is connected to the same motor that develops the suction. You have to find that area and either pull the belt over the motor and/or the brush. Close it back up and turn it on again.

look for a blockage in the head elbow of the suction pipe or in the pipe itself
check foe air leaks that lessen the vacuum

The answer may depend a bit on the specific model, but it sounds like you're handy enough to have gone in and replaced the clutch. A couple of other thoughts, based in part on doing similar things to my DC07, which I picked up at a flea market and did a refurb on before my wife would let it in the house:
There is the clutch, the roller pin that turns the belt, the belt, the flat spot on the brush where the belt should roll, the brush, and the spots where the brush fits into the ends of the cleaner head.

By 'on carpet setting' I assume that's when you move the clutch to the carpet 'engaged' setting.

When you move the clutch in and out of engagement, is the movement smooth, or are there catches or places where you have to apply a lot of pressure (at least if this is a manual clutch, in some cases it's pushbutton engagement)> If there is resistance or sticky places in engaging/disengaging the clutch, you may need to open things up again and see if the clutch is in correctly.

With the vacuum on, move the vacuum between reclined and all-the-way-up, where it should disengage the brush and switch suction to the handle/auxiliary tool path. Does it turn off the brush and switch suction? If not, then the switch-over may be stuck or sticky and messing with smooth operation.

with vacuum off and clutch disengaged (bare floor mode) can you spin the brush easily with your hand? If you engage the clutch, can you roll the brush with some resistance? Is the resistance steady as you roll the brush through a few revolutions. If not steady, there could be something stuck to a shaft, gear, or belt that you'll only hear when those parts are moving.

A lot of hair or other cruft built up on the roller can make for vibrations, especially if it's stuck to the roller lopsided. Clean it up.

Finally, the brush has non-symmetric fixtures at the end that may not be at all obvious for how they fit into the ends. I know from experience that you can seem to get the brush head all back together when the brush ends aren't in fact seated properly. The system doesn't like this. May take some staring, but open the bottom up (vacuum off, of course), and see if the ends seem to be seated cleanly into complementary shapes.

Good Luck!

It'll help if you clarify if this is no power at all - the motor makes no sound, motor is on but no suction, and/or suction but the brush isn't spinning. If the latter, then there are a couple of possibilities. The bar should stop spinning when full upright, where the system will route suction to the handle for accessories. Try reclining the handle. there is also a switch for the brush clutch to engage (see the manual
http://www.dyson.com/medialibrary/files/dyson/support/downloads/uk/floorcare/manuals/DC25_Manual.pdf ), including page 8. The brush will also stop if there is a blockage.The checklist for clearing obstructions and resetting the brush bar (!) are shown on page 5 - there's a switch on the back to press twice, probably re-engaging a breaker.

Good Luck!

the motor spins around a steel shaft that sticks out on one end, and the belt is driven by that shaft, belts are easily damaged and broken, meaning roller wont spin. Often a burning smell of rubber is noticed after motor shaft spins on stuck belt. cause is always hair./string/something at the three contact points. motor shaft and belt, roller and vacuum body become clogged. become familiar with taking cover off roller/belt, and cleaning Checking moving parts roller/belt come out easily and simple to clean...keep spare belts.

If you suspect the switch, unplug the unit, wire around the switch, and see if the motor runs.

You either have the wrong size belt, most likely too small, or you have installed a new belt incorrectly. When you open the cover on the bottom of your vac, you will see the pulley that moves as you flip the switch from floors to carpet. The belt does not go around this pulley. When activated(carpet) the pulley puts tension on the belt causing it to turn the roller. When switched back to hard floors, the belt looses tension allowing motor to cause suction but disabling roller. Also, the dial on top of vac is only to direct suction, not for disabling roller

maybe the belt is broken. clean the unit and replace the belt.

sounds like the brush belt is worn or torn