Npc's do not take any damage from me and won't die

For some reason, any npcs that spawn on the map side cannot be harmed be me but npcs that i spawn can be. for example, if i were on a map such as Gm_missionkillbreen where all the combine where place alread. not even a rocket or super rocket lancher can harm them but they are able to kill eachother. Whats going on here? Plz help. I wana kill some zombies