Some were mounted in odd positions...so I recommend just removing one of the screws first , and rotating the top of the board AWAY from the magnet, as it needs to sense the field from the ball, not the magnet...if that doesn't work, you'll need to take both screws out and find the sweet spot.
Be careful, though, because that magnet gets very hot very quick if you test too many time in a row...
I got good at testing it....
1) put playfield up, resting on second set of rail rubbers ( so you can access the test buttons)
2) get a spare ball, put it in rear lane
3) go to menu settings, pull up the magnet sensor test screen
4) make sure coin door closed , then push "enter" ( left hand) when rolling ball over magnet( right hand), if all is right, the green light comes on ONLY if ball is grabbed by magnet.
5) if light comes on with ball not grabbed, sensor too close
6) make sure to take breaks if you can't get it right after about 4 tries, and let magnet cool
7) once you find it, put one screw in snug...verify its still right
, if so, put in second screw and verify several times ( remembering to let the magnet cool)
Hard to get a good pic...in a tough spot...patience is the key...
I thank Eric from JJP in helping me through this late one evening, when I was debating on setting the machine on fire...
The good news....it's an awesome effect when adjusted properly....good luck....mark