First, you should have enough spirit for the Ambush skill to work. Second, don't take anything you see in the flash (animated) battle reports seriously- they are buggy. So, in order to be 100% there is (not) a bug, look through the detailed (text) battle reports. And, if you see a proof you are not using the skill when you have enough spirit to do so, please mention your account name and the realm you are playing it, so that the support crew checks it.