Why has new belt on DC 07 broken immediately after replacing

Check the ends of your brush roll. Most have bearings on the end and if one of those is locked up and will not turn it will burn the belt. Sometimes hair will get in the bearing area and will contribute to the bearing locking up.

Replace the Roller Brush. Sounds like bearing in it is locking up and the the belt is slipping and not turning the Roller Brush as it has a locked up bearing. You will need to go ahead and get a replacement belt just in case you have stretched the older belt to a bigger size. I had this same experience with a Hoover and it solved my problems of "rubber burnt smell".

More than likely it's coming from the belt in the power head. Due to some difficulty in taking the power head apart and reassembling, I do recommend taking it in to a shop. Getting the tension of the belt correctly is very important. Too tight and it will break the belt, or even cause the motor to burn out sooner than expected by making it work harder. Too loose and you'll have the problem you're having now, and will have to replace the belt again.
Also, hair and thread or dirty bearings on the ends of the roller brush will result in the belt burning out/breaking faster.

If the belt is hot or burning during use, it could be due to a beater brush, which the belt drives, being bound up in some manner. The beater brush is removed during belt changes and should be cleaned of all debris as best you can. At each end of the beater brush there is a plastic clip of sorts that holds it in the vacuum. Spray some WD40 between the bar and the plastic clips then rotate the clips to be sure they move freely. Reassemble with a new belt and put it to use.
If you find this solution to be of help to you I would appreciate a Thumbs UP rating. Paul

Is the motor operating when switched to "on " position? Is so I would have to assume that the belt is broken to the power brush /head. Did you happen to smell burning rubber at all when the brushes stopped working? Look in your manual for replacing power head/brushes belt! WalMart sells most types or your local vacuum repair facility.. check the Yellow pages and let your fingers do the walking.. Good luck

This model has Brush Control
The Brushbar can be turned off to protect rugs and delicate floors
You've either accidently turned the brushbar off or the belts broken

If it is a smell of burning rubber - most likely your vacuum belt is overdue for replacement.

At this point - remove the brush roller, remove the old belt, clean off your brush roller and remove any string or hair that may be bound up around it (usually around where the belt goes or on the sides), and put on a new belt. Your Kirby should act good as new if your bag is replaced at the same time.

If the brush roller has too much string and hair wrapped around it, many times the brush roller will not spin as fast as the belt is driving it and you will get that burning smell.

remove the brush roll clean the brush end bushings and replace with a new belt

Burning rubber smell means the brush belt need attention. Make sure the brush is turning freely and replace the belt

I had the same problem but I found that I had replaced belt incorrectly. After placing one end of belt around pully, it has to be run behind the "belt disengager" which looks like a 1/2" wide metal tab just past pulley. After placing belt correctly I had no more belt, burning smell or brush problems.