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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1 Answer

Bonus stage start everyday at (random) 10:00AM, 4:00PM or 6:00PM for 60min.

You can see all bonus stage throught the POW screen (3 on the first world, for normal and Hard).

You cannot see Bonus stage 2 before clearing Bonus stage 1, and Bonus stage 3 need Bonus stage 2 to be cleared.

On the second world (unlocked after clearing the hard mode of the first world), there is one bonus stage by day, all day.

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