Guild Wars 2

One possibility, did you go into the water? It is now set up that if you switch to a weapon set that doesn’t have the sigil on it, you will lose your stack. When you go in the water you switch to underwater weapons. If one of them doesn’t have the sigil then the stack is lost.

http://wiki.guildwars2.com/wiki/Sigil_of_Bloodlust
As a result of the April 2014 patch, the Sigil of Bloodlust — and presumably, other “stack on kill” Sigils — have been significantly degraded. If you are an Elementalist, all accumulated stacks are lost any time you change weapons, even if the new weapon has the Sigil of Bloodlust. If you carry dual weapons with Bloodlust Sigils, you no longer get two stacks for each kill. If you are a profession with weapon swap capability, you will retain your accumulated stacks only as long as one of your two weapons has the Sigil of Bloodlust.

Oh wow, that’s so bad.. Should I invest in a Sigil of Bloodlust for my Water weapon set? Or is it possibly that they will change this back, I’m sure this has kittened off quite a number of people.

If they do ever switch it back it might be months, or never. I got one for my underwater weapon myself.