Make sure the ball launch assy (coil and plunger) is lined up correctly so that when it strikes the ball it isn't pushing the ball into the side of the launch lane or ramp.
Reduce the height of the metal ramp as much as possible to reduce the steepness of the ramp. There is some adjustment normally where the ramp mounts onto the backboard of the playfield. Drop it as low as possible.
Even take a video of the ball being shot up the ramp a few times and then try to watch back in slow motion and see if the ball is hitting the launch lane or ramp anywhere and slowing the ball down.