Prologue Achievement Not Unlocking?

I have heard that one fix is to play Gears of War 3's prologue on STANDARD not arcade again. If that doesn't work. apparently Epic scans for achievements once a day, So if the achievement scan was already done it won't unlock until the next day so it may
just be as simple as that.

Lol just played the prologue and it chucked 6 achievements at me.

I'll play. I need the prologue for the 4 player achievement but will help others as well

The same thing happen to me. You have to play the prologue in order to get the achievement.

In theory, you only need to complete the missions that we not properly completed to get the achievement. There might be a level that you didn't complete fully (such as using a rally point). You shouldn't have to play the entire campaign again, and if you've
already complete the prologue, you won't need to do that again either.

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Just a side note, once an achievement has been unlocked, you can update or add a time stamp to it.