Ideas
I had similar problem with all defensive gadets.
First I had a problem with deployable shield, sometimes game register only 1 placed out of 30 per game. Once I was placing those shields I have seen that I get points for deploying defences and then i realised that it registered only then. So I started destroying shileds manualy and placing them again, sometimes it worked, but sometimes I could place one per game.
So when it came to the challange with intercepts I was a little bit worried. Although the begging was getting smooth, gadet was intercepting maximum 3 nades before getting destroyed, after sometime, It started working the same as the deployable shield. When I placed a gadet and the game did not reward me with points for defensive structure, challange wasn't counting. This interceptor could catch 1000 nades and none of them would register, because I think the game didn't notice this gadet even working. Only when I got the points for the gadet it started registering.
I hope it helps a little.
Best regards,
Raczulek