Why?

[Legion88]

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.

[KageTokage]

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.

Possibly the dumbest thing about this fact is that you aren't actually considered be in combat until you register on the enmity list, meaning Flamethrower won't generate heat among other annoyances.

[alimdia]

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.

The enmity UI can suffer from desync, I've done S ranks with too many people where I never got any enmity and was considered out of combat the entire fight, but I still got rewards. Normally you don't get any rewards even if you're in a party if you don't contribute in any way, so the server is properly receiving my contribution information, but it can't send it back to the client properly and keeps me out of combat.