It's a pain. Things happen so fast, you can't really see what's happening to it, and it's just in a hard area to see anyways.
I had the same issue. Ball would sometimes not make it all the way up, and sometimes roll back in the subway like the OP shows. I tried flattening out the subway, but it still happens once maybe every 100 tries. So not very often.
For me, my issue is due to the subway mounting tabs breaking off. A previous owner riveted new metal tabs to mount, and the ball would hit one of those rivots on the way up, causing it to lose momentum, or shift the momentum so that it just had no angle to continue through the ramp.
I sanded the rivot down until it was nearly flush with the plastic. That helped for a while, but then it started to have issues again. Then I discovered the way the subway was mounted was shifted over just a tiny bit. So the ball was also hitting a lip on the subway itself. So I sanded that back.
I'd start by rebuilding the coil first. Replace the core, the cradle, and the core sleeve. If that doesn't fix it, inspect the path for any obstructions. Any overhang of anything. Even the smallest, insignificant obstructions. Then, try to take a close look at the wireform and you may need to slightly reposition it. It can move a couple mm's, and that's most likely all it will need.