I've been playing with two other friends over the past few months and we've analysed the problem to the following extent:
When grouped, there is often a problem where one of us cannot get a quest bag item to drop, no matter how many times we repeat the kill.
The third member doesn't get the drop at all. We have to ;
- leave them to kill the relevant mob themselves without helping them, often standing far away, or,
- we kick them from the group and they kill the relevant mob themselves and get the drop, then we re-invite them to the group.