The problem of “Wipe off” occurs when grass or weeds brush the bottom of the sweeper and the debris that is attached to it is pushed backward on the bottom of the sweeper.
If the debris gets pushed to far back and out out of the magnetic field of the magnet, the nails or debris you picked up can fall off. This can happen with many magnetic sweepers that have a magnet inside the housing where the magnet does not fill the entire housing.