Try cleaning the wireform on the under playfield ramp and also adjust the little metal slope piece where the under playfield ramp meets the playfield like the other posters mentioned. With a lot of play this piece bends down below the playfield and makes it harder for a ball to come out smoothly. I would guess that while you are in play other solenoids are going off, so the under playfield ball launch doesn't get full power and the ball doesn't make it out. When you test the coil is firing by itself so there's no problem. The spring steel flap cover really shouldn't/doesn't have much tension at all.