If corpses are left after battle in areas where all enemies must be defeated to advance, existing corpses prevent new waves from spawning and gates from dropping until they vanish. This bug exists in both the test and regular servers, and is most noticable in the Operation: Crimson Hammer and Return of the Axes of Evil(TS) missions
Bodies quickly dissapear when no mender is around on regular levels.
At least thats what i always noticed.
Might be mission-specific?