Saying something will not work on you is basically saying you're immune to it, at least in this sense. There isn't really a middle ground to being composed (at least not that I know of). When the composer is used on someone it either composes them, or it doesn't. I know that John was essentially given a vaccination, metaphorically speaking, against being composed, but I don't see how being inoculated is a 'far cry' from saying "It [b]will not work[/b] on my new form," which is a pretty definitive way to say you're insusceptible to being composed. I am very interested to see the explanation we'll get as to why he was composed.