I'll put an guess that that the OP's version of the stem - despite the stock pictures of similar stems with a wedge design - is an expander type.

If so, it might have been tightened so much at one point as to bulge the steerer tube and the bottom of the stem with it. Hence, even though the expander is loose, the stem and steerer might be mushroomed ever so slightly, making it wider than the tube that it needs to come back through.

I'd recommend clamping a long pipe where the handlebar used to be (preferably of a close enough size that you can tighten the stem onto it), flipping the bike upside down, standing on the pipe - preferably one person per side - and spin the fork from the front wheel while pulling up with all possible force on said wheel. Keep as much lubrication dribbling down the bottom of the fork crown as possible as the friction heats everything up.

-Kurt