We have killed Grand Armsmith Korin three times already and the notes and manual have not dropped for Kael Partisan The Ice Moves.
I suggest you have everyone check their inventory. Dont just look - use the find feature. It might not drop if you already have one.
Yes, but did they have it from a prievous attemp. Do you have group members who are not in the task ?
I have seen the task bug before. It acts weird when people who weren't in the group when the task was gotten. Dropping and getting a new one solved the problem.