Why does the brush bar spin for a few seconds then stop?

It's good that you have cleaned the beater bar and the end of it, as this is often the cause of the problem when the beater bar stops spinning.
If cleaning the beater bar and end of it did not resolve the issue, there are a few other things you can try:

1. Check the belt: The belt connects the motor to the beater bar and if it is damaged or stretched, it can cause the beater bar to stop spinning. To check the belt, turn the vacuum over and locate the belt. If it is damaged or stretched, you will need to replace it.
2. Check for debris in the brush roll housing: Debris can sometimes get stuck in the brush roll housing and cause the beater bar to stop spinning. To check for debris, remove the brush roll and look for any foreign objects that may be stuck in there.
3. Check for a clog in the hose or the suction path: A clog in the hose or suction path can also cause the beater bar to stop spinning. To check for a clog, remove the hose and use a flashlight to look for any blockages. If you find a clog, use a broom handle or similar object to carefully remove it.

If these troubleshooting steps do not resolve the issue, it is possible that there is a problem with the motor or another internal component of the vacuum. In this case, it may be necessary to have a professional repair it.

all battery vacs work on the same principle
so you have to clean out the hose behind the brush right up to the canister.
Hoover air stick has the same problem so they changed the head so that a removable section can be taken out the hose pulled up and a bottle brush can be used to clean out the hose to the canister and behind the brush
You will have to clean out behind the brush up to the canister to fix the problem

Did you ever figure out what was causing it? I’m having the same problem. Dissembled and cleaned it completely, but it’s still doing the same thing.

Replace the brush bar you'll have new brush bar better clean than old brush with new bearing which aren't available or worth the bother fitting, ps keep the filters clean makes a real difference to sucking power.

Sounds like the motor in the brushhead unit has died

Hi Jyn, it's Jack at Dyson.

The rotating brush bar in DC23's Turbine head is powered by the vacuum's airflow. From what you've described, it sounds like there may be a blockage somewhere in the machine that's affecting the performance.

To locate and remove any blockages, please take a look at our online Support guide and follow the steps shown on each of the pages: http://www.dyson.co.uk/support/dc23/dc23-animal/my-machine-is-not-working/check-telescopic-wand-step-1-turbine-head

In normal use the pre-filter should be washed at least once every 3-6 months, using cold water without detergent. Even a filter that appears to be externally clean may still need a wash as fine dust particles may be contained within the filter material. The correct method is shown in the video attached. Make sure you allow the filter to dry completely for at least 24 hours before replacing it.

We would advise against attempting to modify the Turbine head in any way, as this could cause damage not covered under guarantee. If you'd like some further advice from our experts please feel free to give us a call - our contact numbers are: 0800 298 0298 (UK), 1-866-693-9766 (US), 1-877-397-6622 (CA) or 1800 239 766 (AU).

If you know some one with compressed air blow out all dirt particles best as possible then get a 3 in 1 oil or sewing machine oil drip along the motor shaft and turn by hand you will be able to free the difference once lubricated it should only take a few drops per side . let set for awhile and repeat . then put her together and see if motor speed returns to normal . also check turbine wheel for slipping on shaft.

Shane:
Unplug the machine and lay it on it's back with the handle release in the release position (the vacuum head should be laying flat also). Remove the foot plate from the vacuum head. Manually turn the brush roll (called a beater bar). The beater bar should turn fairly easily and the belt should turn the motor armature shaft (now would be a good time to check the inside surface of the belt for grooves or damaged spots which would cause the armature shaft to slip on the belt). If the beater bar has too much resistance to your turning it, I would suspect that the bearings at each end of the beater bar are fouled with hair, carpet fibers and string. Remove the beater bar from the machine and manually spin the motor armature shaft (where the belt contacts the motor) to see that the motor armature shaft spins freely. Remove the bearings from the end of the beater bar (they won't come off easily so you may have to use a prying tool such as a screwdriver). Try to spin the bearings. If they are fouled, dribble some rubbing alcohol into the bearing (I use a spray bottle... it's much neater) and turn the bearings while keeping them soaked with alcohol until they spin freely without ratcheting. Drop some 30 weight engine oil into the bearings until they are saturated, then clean the outsides of the bearings with a clean rag (no alcohol) until the outsides of the bearings are completely dry and no oil drips out of them (don't want the oil to attract dirt). You'll probably need to replace the belt. Reassemble the machine and it should work for you.

I had this problem with the Dyson Ball vacuum cleaner. This model does not have a motor head, but drives the brush bar with the air flow. The brush bar was spinning very fast and the head was not picking up any dirt. The outlet from the brush bar chamber was blocked and the entire air flow was being drawn through the turbo, making the brush bar spin very fast. The solution was to remove the brush bar and and clean the outlet from the brush bar chamber. Note that fluff building up on the filter disk (at the side of the turbo chamber) for the brush bar turbo can reduce the brush bar speed.

this link fixed my prob :
http://www.robotreviews.com/chat/viewtopic.php?t=10628
and just in case the link doesn't work, basically, you have to take out the whole red brush-head assembly unit by unscrewing the 4 screw screws that hold in down in either corner; then find the side of the unit that has the gearing inputs, and unscrew all the screws until you get inside and clean out all the hair...
That should do it :)