Which double hit? The aftershock or the combo finisher? Aftershock is the neutral heavy attack that begins with a sweep then an AoE slam, the other is the overhead after a light attack or sprint/dodge attack. The first one double hit should go off every time. The second one double hit requires you to queue up the second strike manually every time. If it isn't going off on majoris it is likely because you are being forced to stop the second strike to parry and not landing it.

Without a more detailed explanation of the problem or screenshots/video that is the best I can offer.