One of the main issues with any type of vacuum that can reduce cleaning and suction power is when the roller becomes full of debris. Hair and carpet strands can get stuck in a roller and cause the roller to stop moving at the appropriate speed. This can also get in the way of suction. Many times you will notice an odd noise when you turn your cleaner on.
If you have noticed a loud noise, if your Kirby is difficult to push or it is not cleaning well, troubleshoot for a roller problem. Turn your vacuum off and unplug the cord. Push down the adjustment lever and take off the hood. Now you will be able to get to the area that holds the belt. Check the belt for any wear or extreme tear. A stretched out belt can cause these issues as well and they are easy to replace.
The roller may need to be taken completely out of the hood to be checked. Look for any cracks and remove any hair or upholstery strands that are wrapped around it. If the roller is not inserted correctly back into the hood, the vacuum will not function properly. Be sure to look at your manual if you do not know how to put it back in.
Lastly, you should check the bag of the vacuum. A small leak or tear can drastically reduce overall cleaning power. Changing a vacuum bag is simple. A bag that is caked with dirt will also need to be replaced for full function cleaning.