Ok, so I wanted to unlock the MG14 Parabellum Suppressive, and started with the Airburst Mortar. I got a kill, but it didn't appear to register. I then switched to taking out the aircraft with an LMG, which I achieved and it registers as achieved.
I've now switched back to the mortar challenge, got a kill, but again, it's not registering. What's going on here?