Has it been cleaned? New sleeve? If metal sleeve, cleaned?

That’s where I would start.

If metal sleeve, replaced with plastic sleeve?

There's never a reason to keep the metal sleeves in coils assuming you can get an equivalent plastic sleeve unless it's something that could heat up a lot (and melt the plastic). Any momentary solenoid isn't going to need it.

For the OP are you sure the ball trajectory is correct the arm that shoots it can wear over time (the tip) and you can also aim it slightly - if it's directing most of its force into the ball guides it'll probably never make it to the upper playfield.

Game definitely has the correct coil? i.e. someone hasn't previously changed it for a weaker coil? Manual says the part number is "AO-27-1300". Does the coil wrapper show signs of being cooked?

After cleaning, sleeve replacement, removing windings of coil would increase power. You could remove say 5 wraps and retest. Caution though, the game should function with original designed coil.