Odd....a long shot, but if the internals of the lock barrel have become well crudded up perhaps one or two of the sliders have failed to return and are jamming the lock. Do you still have the the little hinged protector flap in place?
If you haven't already, get some WD40 in there, not that it will be easy with the key in the way. If you free it off next job would be to whip out the whole barrel (there's a howto
) and clean/check it over