You did put the key in the font at the entrance of the dungeon?
You accepted, started the dungeon and you spawn with your group behind the barrier with a countdown?
If not, then you did not actually use the key but started a regular +0 instead.
Sometimes pugs just leave and there’s nothing you can do about it, hense why I always recommend finding a guild or group of friends to run them with instead.
Since you failed the key, it will go down and is now +6.