I'm with Oddjob.

Time can really help. I've had stuck parts not budge after a quick blast of PB, then I'd give it another blast, gently tap on the part with a hammer, then come back a few days later, more PB, more tapping, etc. And as Zandoval suggests, there's no reason you can't apply the stuff from both ends of the steerer tube.

In the case of one seemingly intractable stem, I think I gave it a week total of this treatment, then set a chunk of 2x4 on the top of the stem to act as a protective pad and banged it down with a 4 lb hammer. Once it goes down a bit, it's free and comes right out.