I assume you have a door or debris leading into the next zone? If so, make sure the trigger for that door or debris has the KvP script_flag / enter_zone_2 and for the next zone it would be enter_zone_3. Make sure that this is added in your map's gsc file too. It should look something like this in the file under the dlc3 zone init();..
add_adjacent_zone ("start_zone", "zone_2", "enter_zone_2");
add_adjacent_zone ("zone_2", "zone_3", "enter_zone_3");
The only other answer I could think of, is to make sure nothing else is highlighted in radiant. Once that's done, highlight your door or debris, and then highlight the zombie. Once they're both highlighted hit "w" and make sure to start with the door first then the zombie second. This will target the zombies to spawn once you've bought your entrance in. Hope this helps.