The swamp problem could be that the machine is not level. seen this a few times. It may also be installed badly. The brackets that fix the swamp assy to the playfield must be flush with the playfield. There is a post and nut in the vicinity, and sometimes the swamp bracket is raised - this causes an effect where the natural fall is removed so the ball sits there. Also, clean the swamp assy - regrain it with scotchbrite.

Follow the above steps and it should work fine.

Most of these issues are fixed during the restoration.