If by any chance you're using Sibelius 5.2 with the latest template, the ";inverted mordent"; symbol has been applied to the key directly over the '8' on the 4th keypad layout. Info on this symbol and its location is detailed in the template ReadMe file on pg. 18-19.
I know I stumbled with this initially until I figured out where that symbol was located on the keypad. The symbol and notehead in Sibelius have to be precise in order for the correct sound to be triggered.