Funny. That unjam key has always worked on every typewriter I have used it on. Takes more force than expected, but it worked.
I always use it deliberately and it never became a habit, I think because it is such a random feature across makes and models and years and doesn't have a standard marking so it requires thought to remember to use it. If you're always using the same tw, then it's easy. In using so many different tws, I have the habit of reaching up to clear a jam on everything so I get the job done way quicker than determining whether and where the unjam keys is.
It is a nice feature for manicured typists who type all day long and have to keep their fingers and clothes clean. And, again, they're using a single make and model.