There are a total of 100 places in the game where Oblivion gates can spawn. Of those, ten are guaranteed (the Kvatch gate, the Bruma gate, the city gates, the Great Gate, and the Fort Sutch gate), and the other 90 are random.
Out of those 90 random gates, only 50 are permitted to ever be opened at maximum through the entire game (which means the all-time maximum number of gates you can ever encounter is 60). Closed gates do not reopen, so the Oblivion Gates Shut statistic is a pretty good method of tracking how many gates are present. However, that will not take into account how many gates are actually open, to get that statistic, you have to be on PC and have access to the command console. If you do, you can type in the command:
sqv mq00
and you can then see exactly how many random gates the game has opened. If the number of open gates is 50, you have hit the cap and the game will not spawn any more. Ever.
Otherwise, at that particular point in the MQ, there is a 50% chance that passing by a random gate spawn point will trigger that particular gate to spawn.
Basically, either you've exhausted the random gate spawn allotment for that character, or you're just REALLY unlucky.