Illaoi W Bug

I'm not sure whether or not Illaoi's W should work on towers, but sometimes it does, and sometimes it doesn't. For instance, earlier in the game i will W a tower, and it will only do basic auto attack damage, but later on whilst attacking an inhib or other tower, my W will do the additional damage that it should do. Not sure if this is a bug or if this is meant to happen? Thanks- RiChDiNoS