Doors get stuck because they're out of alignment with the frame. Usually you can fix this problem by adjusting the position of the door within its frame. For example, if the door is sticking on the upper right side, chances are there's a visible gap between the door and the door frame on the upper left side. In this scenario, you'd need to shift the door to the left and close up some of that gap.
I'd caution you that this is not a do-it-self project for your mom; she should get help adjusting the door.
Door frames and doors shift, settle and re-settle all the time due to changing weather conditions. It doesn't necessarily mean there's anything wrong with the door or the building. Sometimes these occasional adjustments need to be made to keep things working smoothly.