Why can't I kill stealthily?
- Non-dagger weapons are not silent. If you are using a sword, they are hearing you.
- Do you have a follower? They are notoriously loud.
- There's a chance that the victim dropped a large, metal item (weapon, shield, etc) that landed somewhere unfortunate and skidded along the ground noisily. I can't imagine this is the case every time, though.
Are you fighting Forsworn? They have heightened senses and are harder to sneak kill than other human characters.
Make sure you're wearing equipment that's been enchanted by muffle. Also be cognizant of the shadows. Walking onto any surface with light reduces your chance to successfully sneak on a NPC. Sometimes even the slightest inconsistencies can easily cause detection. Walking over pressure plates, weapons, or any miscellaneous objects are all ways of making noise. As a fellow Nightingale, walk with the shadows.