There's a certain type of soldier that opens each door. First area should leave about 2, 1 or each door and the last one is I think. in the last group that spawns. I think they are the "green eyed soldiers". But the thing is, when each new group spawns ahead of you, don't go killing them all, run up and grab the proper soldier first to open the door then kill this group and move onto the next. They need to be alive and I recall there is only a certain number of these guys that can open the doors so maybe the last one got killed by accident without you knowing.