It's a pretty basic cross block safety. There's a arm on top of the trigger. The safety pin has a narrow spot that allows this arm to move when the trigger is pulled. You push in the safety and a wider part of of moves below this arm, blocking it's movement.
Check that the arm on top of the trigger isn't bent. If someone pulled on the trigger really hard it could have bent up. However the trigger is plastic, so there's no real good way of bending it back.
David Johnson, AKA Fubarius.
Nunquam emere quid potes facere