I had same problem. Solenoid would fire but ball would not always make it up to the metal ramp and out to playfield.
On my game, one of the legs of the wire form had a broken weld at the base. The loose leg would absorb the energy and the ball would fall back down. I put a hose clamp around the four wire "legs" about 13 years ago and it has not been a problem since. The clamp is hidden by the plastic. Fyi