I also have a 116.23512 canister, and am having the same problem. The difference with my situation is that one of the end caps on the agitator assembly was imobile. I cleaned it, even removing the end cap [it seems they do come off]. The end cap is now mobile, but does not spin as freely as the other end cap, and appears to be slightly misaligned, as though it had be banged against something, or even perhaps overheated, which is possible if the power head was operated too long with the assembly not being functional. If you pop the end cap off, you can see a round metal thing it covers. Mine had a faint burned smell, similar to the smell you get when you run the powere head, and the plastic on the inside of the end cap appears to have been very slightly melted. So, I'm not really sure wether debris initially caused my problem, or whether it is an electrical issue which cause some damage to the essembly, rendering imobile.
If anyone has any thoughts, they would certainly be appreciated.
Thanks
Dottie
February 2005