Guild Wars 2 has bugs. All MMOs have little issues here and there, where the code reacts differently to the millions of people playing than it is expected to. One of Arenanet’s stated goals is to find and fix bugs wherever they can, and to do so quickly so as to have the fix available for the next patch or even, for game-breaking bugs, to be able to hotfix them.
One such bug Arenanet has been getting repeated reports on is a bug with looting. Lots of characters would be involved fighting enemies, particularly the larger champion-scale enemies, but some of them would not receive any of the items those enemies are supposed to drop on defeat.
Initially the suspicion was that the loot tables for those monsters were broken, but eventually they determined that what was at fault was the formula determining who receives loot. It is a different formula than the one determining who receives experience and is much more strict.
Having finally isolated the problem, Colin Johanson has posted on the forums about the issue and confirmed that the problem will be corrected in the upcoming patch, launching with Flame and Frost: The Gathering Storm. Just one more step in the right direction for Guild Wars 2.