Rarely the game bugs so that you can't loot one of the bodies, but it shouldn't happen all of the time. Do you have any add-ons running? If so, you might want to turn them off and try again. Also, check to see if you are getting items in your inventory anyway, in which case you might have "Autoloot" turned on.
Also be aware that if you are way over leveled for a certain zone that you may not be able to loot the killed NPC's.
Lately, the game has been slow to recognize that the combat is over.
I reported, and still see, cases where the server and client are in a bit of a disagreement. This includes cases where the corpses are not marked for looting right away. It also includes cases where the corpses say they can be looted, but they will not open.
Hopefully, this and other problems that I, in my ignorance, consider to be linked will be fixed before March 17. I suspect it is a server attention deficit disorder issue since latency reports good.
Edit... oops. The new forum design does not tell me category before I open the thread and I forgot to look after I opened it. My comment is related to Live, not PTS.
If you are significantly over the level of your enemy you are unlikely to get loot from them, except for the level-independent materials as mention in the above response.
Also, if you run into the bug of an enemy that is highlighted for loot but cannot be looted, a workable solution is to kill something else close to them (even a vermin) and loot them... the loot grouping will allow you to simultaneously loot any kill within a close radius in the one action, including the bugged one.