1: No this is not common at least on any WOZ I've ever played including my own. I will very rarely have a ball slow roll back out if it was weakly bumped up there by a pop or something but I'd suspect you still have a wire in the way, the balls are magnetized or the game is too flat.

2: There is a dedicated test mode for the Monkey and part of that is indicating when the magnet sensor is triggered. Put the game in that mode and roll a ball slowly towards the magnet then re-position the sensor (very small board attached to the back of the backboard behind the magnet area). Make sure the connection is solid, etc. and also remember to not run the test for too long at a time as the magnet will get very hot quickly. There are multiple posts about this in the WOZ club thread so search that for more detailed information and expert advice from LTG.