if you loosen the bolt quite a bit, and it wont pull out, get some PB blaster or Liquid Wrench and spray it in there and leave it for a few hours to a day.
this will help.

the way a quill stem works is that bolt screws into either a wedge shaped thing or a tapered cone shaped thing that will fix the stem in there once you tighten the bolt.
allot of time, since quill stems are "old school" so to speak, they've been in there a long time and time and water will rust it up down in there, and that wedge that the bolt is screwing into becomes stuck. one thing that has also worked for me is unscrewing the bolt out enough that it sticks up but is still screwed in some, then i take a rubber mallet and whack it on the bolt a few times. this will push the wedge down and unstick it.

good luck!