The spot I always miss is at the top of the needle bar swing arm, where the mechanism moves to make the zigzag. There should be an oil spot there. Take the needle out, hit that spot with Liquid Wrench, then run it like heck on the widest zigzag setting its got. Some of the Liquid Wrench will dribble back out, probably an ooky brown. Clean up, hit with machine oil, run it again, see where that gets you. Alternatively, perhaps it's something on the 'inside' end of the ZZ mechanism that needs oil?