It has been my experience that these beasts fail in three typical ways:

a) they are so seldom checked that dirt accumulates under the leaf and cements the door in place (!!).
b) the plate rusts to the magnet.
c) see a) above but insert carpet.

All due to systematic and systemic negligence. Under my watch, such devices are tested weekly, typically on Friday at office (or school) closing. I have never seen residual magnetism sufficient to hold the door. Neither plate nor magnet has enough mass for that.

Peter Wieck
Melrose Park, PA