How do I unlock more bonus worlds in Metal Slug Defense?
In Metal Slug Defense for iPhone and Android the game sporadically tells you about a bonus world which will be playable for around 5 to 60 minutes.
After freeing every Prisoner of War (POW) in each level of one bonus world the world becomes permanent playable and will not disappear anymore.
My problem is that I am not getting new messages about the next bonus worlds (or: no new bonues worlds become playable). Is there any requirement I have to meet? I already finished World 2 (hard mode) but do not have all POWs freed by now.
Any ideas?
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