Akira's m-SgPm back crumble against TR

Am I the only one who can't get m-SgPm to back crumble a TR? I hit the m-SgPm just like I would d/f+K to backstagger, but all it does is hit them in the back of the head. The opponent goes into a crouching animation and the SPoD will not combo. Actually, nothing will combo because it isn't a backstagger and it isn't a back crumble. It is a m-SgPm to the back of the head and that's it!
I get the same animation when I use m-SgPm on a QR as well. And trust me, I know what a back crumble is. I've seen Akira's m-SgPm after SE and Lei-Fei's lunging punch back crumble plenty of times. I just need to be enlightened as to how the m-SgPm is supposed to back crumble a "crouching" opponent during a TR/QR when, normally, hitting a crouching opponent in the back [of the head] doesn't back crumble at all (normal as in m-SgPm on a neutral, crouching opponent in practice). What gives?