Why does my physics model refuse to move when hit from the front or back?

I made a physics model of a Chuck Mangione cardboard cutout (from King of the Hill’s Mega-Lo-Mart).
Anywho, I’m having trouble with movement. When I shoot it from the side, he flies away (as expected). But when I shoot it from the front or back, it doesn’t budge. I’ve tried lowering mass and increasing inertia, but it stays stationary when shot from the front or back.
Does anyone have any idea of why he’s not moving or how I can fix it?

I bet that collision model is orientated wrong.

Wait, I think I’ve got it!
This whole time I’ve been shooting his face in with the Scout’s shotgun. Just now, I switched to the pistol. When I shot him in the face, he started doing backflips!

So for some reason, he doesn’t even flinch when hit with a massive shotgun blast point blank, yet he goes crazy when a single pistol bullet lands between his eyes.

Anyone have any theories on this shoddy shotty non-response?