You are probably hitting a target with a shield (hardened ward, healing ward etc.). Shields can never be critically hit, even if your crit chance is 100%.
tribbsb16_ESO wrote: »Have you tried it with other skills, maybe it's ambush that's bugged
You have to be cloaked when you hit the target. It's not like might of the guild where you get a 5sec window after cloak. You can be in stealth hit invisibility snipe and not get a crit. But if you snipe first then right before it lands on the target hit shadowy disguise it should crit.
The tooltip wording doesn't really match what the ability does.
When you activate the cloak, you gain a buff called Precision. If you attack while this Precision buff is active, it should crit.
However, Precision has a very short duration--same as the cloak itself. So you must attack before the cloak wears off--if your "next attack" happens after the cloak has worn off, it won't get Precision.