Honestly, it's probably something simple like the boss not being where it shows that she is. It happens to Dranos Validor in Cradle of Shadows, he's super buggy and slides around the map, so if he dies when he's being all weird, his body will appear to be in one place, but the server will think he's somewhere else, and you'll get the message "You're not close enough to loot".

If it's not giving you that message, could be that you just need to roll-dodge, switch weapons and do a heavy attack while also singing Britney Spears' Toxic backwards. That'll do the trick for sure.