Speaking as layman, it sounds plausible that a hit on the back would work while the target missile is ascending, but not when it's already descending towards the target.
I'm far from being an expert, so everything I say could be wrong and I am aware of this! But I assume that because of wind influence, etc, I suspect that a rocket needs to have manoeuvring capabilities to reach a specific target, even on their descend. I also assume that works with little wings on its side and GPS. Destroying those wings or the GPS might be enough that it lands tens or hundreds meters aside from it's designated targets. Maybe even kilometres, but I really don't know enough about that to say.
Speaking as layman, it sounds plausible that a hit on the back would work while the target missile is ascending, but not when it's already descending towards the target.