Sure Alex mentioned they'd rejigged the Boss fight payouts

Arh, here it is "
- the drop rates for Boss fight rewards have been adjusted as follows:

Armageddon: from 1% to 3%
Epic: from 5% to 7%
Rare: from 20% to 25%
Uncommon: from 74% to 65%"

http://forum.warzonefb.com/showthread.php?t=1906

Might explain why they are not dropping like they used too