The SAM operates by pulling the strike plate against the magnet face when the door closes, seating the conical buttons on the strike into corresponding machined holes at either end of the magnet body. If the buttons do not seat, the lock will not hold. There are three potential causes that can produce a failure of the buttons to seat. First, the mounting alignment between the strike and the magnet body can be off so that the buttons don't line up with the conical machined holes in the T brackets. A misalignment greater than 1/8 inch end to end or front to back will cause engagement failure. You can normally visually detect such an alignment problem. Watch the strike closely as you very slowly close the door. You should see it "try" to move against the magnet body but note that the buttons are acting as stand-offs because they are not lining up with the holes. In some cases, this problem can be corrected by adjusting the door but re-mounting the unit properly is often required. The second possible cause is that the gap between the magnet body and strike plate has widened to the point that the magnet can no longer pull the strike plate in. This can happen, for example, when the lock is mounted at the top of the door and the door sags downwards which increases the gap. Note that the gap is supposed to be 1/10 inch or the point at which the tops of the buttons just graze the magnet surface. If the actual gap is significantly greater than this, you have found the problem. It can be corrected by either readjusting the hanging position of the door or readjusting the level of the strike (by turning the two strike mounting screws) so as to reduce the gap to the correct dimension. The final possible cause is that the strike mounting hardware has somehow become frozen so that the strike has lost its movement ability towards and away from the magnet body. You can check this by trying the move the strike by hand with the door open. If it will not move, dismount it and clean and lubricate (or replace) the mounting hardware.