Verified answer

You are either not holding the shout button long enough for all three words to be shouted and thus have the spectral armor on arms (first word) only. The other two words extend the protection to chest (second word) and head (third word) and you need to hold the shout key down until all three words are spoken. Or you have hit one of the bugs where only the first word works and the effect is only on the arms. The only fix I know of for the bug is to load an earlier save and try to learn all three words again.

Dragon Aspect

I think it depends