I can't unlock atachments at any weapon

Hey guys ivГ© played this game like one hour and only with a single gun i made 70 kils per 41 deaths and when the match finished i went to my classes to modify my m4a1 but i couldn't do that cause i hadn't got a single kill with that gun and in another server with ump i've made some kills and again it say that i hadn't make any kill with it.I don't know if it is important but for 3 rounds with m4a1 when i've died i was respawned with ACR instead of m4a1 but when i shot with it it sounded like m4a1.Thanks if anyoane wants to help!I forgot to say that is about modern warfare 2 if it is someone who doesn't know.