Is the subway plugged up with something?

When i had this problem, i checked where the ball got stuck with an endoscope camera.

Do you have the machine leveled? When I got my Addams Family I set all the legs low which made it easier for kids to play but it wasn't level according to the little bubble. At times we would have to bump the machine (not too hard) to get the ball to to the swap. I've set the back legs up high a few months ago and this hasn't been a issue since.

Make sure it is leveled as suggested. Check the box from under the playfield and make sure it is mounted properly, all the screws are there and the box is not bent/deformed.

Are you sure it's stuck in the box rather than in the swamp? It is just gravity fed to the swamp. If the solenoid fires, which means the switch is sensing the pinball, it sounds more like it may be stuck in the swamp.

I had this problem as well. Setting the legs higher in back and making sure it was level has done the trick. It used to get stuck though every so often and you'd have to slam the right side of the cab with a number of good whacks to get it to roll out of the box to the kicker down there to shoot it up to the swamp. I believe I also had that kicker's white plastic plunger snap at some point, in which case you probably won't get the ball out of there at all until you replace it. Sounds like your problem though is not having enough of an angle on the machine or maybe leveled to the left a bit so the ball gets hung up at the edge of the box hole where it feeds out to the kicker.

^this^

My BSD long ago had an intermittent issue with the ball not getting where it is supposed to after dropping into a box under the playfield (just like Addams). When I finally lifted the playfield to check it out I discovered that only part of the screws were still holding it on and it was dipping just enough to not allow gravity to do it's thing.

That would cause it to stick and not activate the thing kickout coil. Clean the plunger and put a new sleeve on it.

A cell phone taking a video does wonders here as well. Only disadvantage is you get to see what happened after the fact.

Are you positive it's hanging up in the box vs in the swamp. If it's the box just make sure it is mounted properly and the exit to the swamp is aligned properly. You can have all of the screws tight and still be mis-aligned. If it's the swamp you can get a swamp accelerator in the following link or there's a 3D printed one available. Don't remember where, but there should be a post on Pinside about it.

http://www.leesparts.com/products/addams-swamp-accelerator.html

So I have had this happen before. And I had to fix it by bending the metal box back out a little bit. Somehow mine closed the gap just enough to make the fit too tight for it to pass through the side up the upkicker assembly where it enters. In fact I had the issue happen to where after I fixed it, the switch in the upkicker was being tripped and holding the ball up as well. After I fixed the 2 issues, I have had no problems since. Also I would check to make sure there is no gap between the box exit, and the upkicker entry.

I had this same issue, and in my case, the box had dimpled from thousands of ball drops. Enough that a depression was holding it instead of letting it roll through the narrow opening into the coil mech. Banging the metal back flush fixed the issue. Definitely check level and mounting screws first though.