Coachman 675 Door Magnets

I have a Coachman twin axle 675 caravan. I have a problem with the door magnets, The dealer has looked at this and can't understand why they don't hold the door to the side body. I had the door replaced due to some hollows appearing in the top of the door since then the magnet will not hold the door open, i've had new magnets when you put one to touch the other in your hand or one against the door or the side of the van they hold, but when atttached and you close the door it doesn't hold the door closed. Never heard of anything like this. Does anyone have any idea what is going on. The dealer came to the house and spent an hour trying to solve this mystery but went away not knowing how to solve this.

Thanks  Brian