I often find that my sneak attacks aren't triggering properly, even when all conditions are met.
My character is within the correct distance.
I have tried dis-engaging and re-engaging the opponent.
At no point did the sneak-vulnerable opponent switch its focus to my character.
I was not able to land a sneak attack on any opponent for the remainder of combat (not only that flurry).
Here's the example. I got a sneak. The character moved to another location for better targeting. Suddenly, no sneaks.
I have noticed this is happening very often against many different types of opponents.
EDIT: Just to clarify, the first attack in a round is not always a sneak. Sometimes they don't fire off, period, even when my character is not and has never been the target for the opponent, and the monster is engaged in combat with another character.