The shaft on the end of the IP has a fine spline that fits into the timer/drive gear(the gear has a weight and spring advance mechanism) I guess it could be possible that these splines have stripped. Is the pump bolted securely to the engine? when you remove the IP you unbolt it from the block and a support underneath and pull it up and back. the drive gear stays in place. Here is a link to an article that may help.

http://engine.articles.mbz.org/diesel/injtiming/

The other articles are good too.

__________________
Bob D.
Parrish, FL
1 SDL, 1 D, 1 TD, 1 Mog