Why am I not getting any crystal?

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.

I found what caused my problem: I was in the week of festivals, which cuts all resource income in half. Since I usually only had an income of one crystal per day, the festival cut it down to zero per day.

I played a game with a friend, and both of us had severely slashed incomes of all resources. This happened week after week, until I restarted from an autosave to see if that solved it. Sure enough,my crystal income went from 0 to 2 and everything else virtually doubled!

Looks like it's some kind of weird bug.:s It really makes it hard to develop your towns. :s