There is something missing and/or wrong in the OP description. There is no such thing as "battered zombie trooper corpses", which the OP claims to have aplenty - the battered zombie troopers just drop zombie trooper corpses, which is what you need and are clear if you have one. You can capture battered zombie troopers, but they are not useful for anything. The more likely cause here could be the research is hidden.
The problem mentioned by Stone Lake can (rarely) occur though. If you successfully finish the mission, it won't spawn anymore. If all the corpses got caught in explosions, or if you sold the corpses by mistake, tough luck. Or if you only captured live ones, because the mod settings won't allow you to kill live prisoners to get their corpses.