Does this happen even in a brand new FLA file? If so, please try deleting your Flash CS5 user-configuration directory:
Mac: boot drive/Users/<username>/Library/Application Support/Adobe/Flash CS5/<language>/Configuration
Windows Vista/Windows 7: boot drive\Users\<username>\AppData\Local\Adobe\Flash CS5\<language>\Configuration
boot drive\Documents and Settings\<username>\Local Settings\Application Data\Adobe\Flash CS5\<language>\Configuration
If it's happening only in specific files, can you post the file?
I delete folders from "Configuration" directory and Bone Tool work.
Some extensions from Tool panel make that Bone Tool not work.
I delete this extensions and now its OK.
The reason you are having a problem, mdsena, is because you have not distinguished your symbols into the library panel. You must first save your symbols as a library item. Then the Bone tool is fully functional. Hope this helps!