Possibilities...
She tried to use the room key on the wrong room.
She put the key in backwards.
She slid the key in and out too fast.
The key "expired" for that day, and she didn't go to the front desk to get it working again.
The batteries on the door lock shot craps.
The card got de-magnetized somehow.
She tried to use the key on a room with two doors. The second door was deadbolted.
It was an elaborate ruse by one provider to swindle another provider for some unknown reason, and she would have gotten away with it all if it weren't for you meddling kids!