The first thing is to remove anything that's wrapped around the beater bar. You probably did this already.
Next take off the cover and look at the end caps on the beater bar. On my DC14, the end farthest from the belt tends to gather lint under the end cap. Use a tiny screwdriver or the pointy end of a nail file to dig out the lint. You may be surprised how much accumulates in that small space. Rotate the bar while doing this. The gap is larger at one spot in rotation, and it will be easier to dig out the lint at this spot.
Once all the lint is out, check that the beater bar rotates freely. Put the cover back on and you're set.
This would make a lot more sense with pictures. I hope you're able to figure out what I mean.