you probably didnt assign it to body node 60 on the nif itself.
you will also not be able to see it in first person, unless you modify all races to accept 1p visibility on node 60 (which may cause problems if the user has some other mod that uses slot 60 for something else entirely, then that armor will now be visible in 1st person which would be bad if its something that is on the head or near the chest).
long story short, i don't think it's worth it, but this is only my opinion.