Why did I die?

[DeathsSilkyMist]

There is no fail message for FD. You need to /con the mobs to check if your FD succeded or failed. They should con indifferent if it worked. It sounds like FD simply failed and you didn't know.

This doesn't apply to your situation, but if you are actively autoattacking a mob when you FD, that can cause FD to fail as well. You need to turn autoattack off before attenpting to FD.