I have premium mop v3 and let's say i kill 2 enemies and both are lootable, i loot one and the loot from the other disappear, so if i kill more then 1 i can only loot 1. If i want to loot all i have to loot as soon as i kill 1 before i kill another.
MoP introduced area of effect looting, where all nearby mobs are looted at once. Sounds like this is what is happening when you are expecting to loot each mob separately.
It is an aoe loot now, I made the same mistake when MoP launched and I was getting batty because I could not loot the other corpses, then I read the release notes and saw that feature.