Heat often helps if they are stubborn. If you have something like a hot air gun, then use that to heat the chuck up first. You can also try putting the biggest hex key you have with the short end clamped in the chuck, then give the long end a sharp downward thump with a hammer, better still put a tube over the key so you can support the far end of the tube, then use something heavy and thin, like the shaft of a big spanner, to tap it downward as close to the chuck as you can. Don't go mad, If a few sharp heavy taps doesn't shift it, then you will have to get some wedges. Don't try and twist it, you will damage the quill guide pin, which runs in the long slot on the side, or you might damage the quill itself.