Are you sure you actually own it? If there's an enemy-controlled (or even neutral-controlled fort / town in the area, all of the mine's production will go to them, unless you've got a hero constantly sitting on top of the mine.
What week is it? Certain weeks can cause resource or creature surplus / shortfalls.
Has an enemy hero plundered / sabotaged a mine? That could prevent production also.
There are also occasions were, sporadically, Crystal production drops to zero due to bugs. If none of the above, you might try reloading the game, which will cause crystal production to resume if it has stopped from the bug.