NPC stops moving when....

Hello everyone. When I spawn NPC, for example zombie / or rebel, rebel is following me and zombie is trying to hurt me. BUT when I go like 100 metres away from these NPCs, zombie / rebel will freeze and stop. I must go 10/20 metres closer to them, becouse otherwise they will not do anything. If you don´t understand my dumb english, there is another example: Combine with pistol / smg is shooting at me, but when I run away (on gm_flatgrass) combine still see me but don´t do anything. I must come closer and then he starts shooting… Is possible to set NPC view distance so they can follow me all over the map, even when i am on the other side of map? Thanks for replies