Don't forget there is a maximum of slots on the enmity list of enemies.
If your not on the list you get no contribution.
You can check it if you are looking at your enmity bar.
It fills or you have an enmity order number? Gratz your on the enmity list and get contribution.
It doesn't fill or you have no enmity order number? Too bad, but you don't get contribution.

This phenomen can be seen by all huge bosses, like S-Hunts, Ixion or others of this type when a huge crowd of people try to kill it.