Why won't my 6 month old husky howl?

Well, she doesn't know how to or hasn't had the desire/need to - yet. Eventually she will start to do this, it's just a matter of the 'right' stimulus and time. Not that this is any comparison but I had a cat once that didn't know how to meow - she used to make little hiccuping sounds. Until, one day I had to take her to the vet and she didn't like being in the car, not at all, and that's the day when she learned how to meow, loud and true. Give her time, their actions usually are part of a series of events where one thing triggers another.