The Door Open sensor needs a clear, unobstructed line of sight to the opening side of the door.
The sensor is reporting either detection of light or a difference in the distance to the door.
Ensure that the Monitor is installed appropriately, to permit the detection. Velcro may be used to mount on the side of the safe, or double-sided tape. Or mounting screws.
To check the placement/installation for your Safe Monitor, view this article: