Probably just bad luck. I've used maybe 15 and have got around 5 exotics, but that is probably just really good luck.
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I used 789 and not even 1 exotic
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You're supposed to use one at a time. They don't stack on usage. But with how many failures there were
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I did use one at a time
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I find it hard to believe you ran 789 strikes one at a time. That's at LEAST 789 minutes 13 hours if you flew through easy strikes. I doubt you were that dedicated to spend 13 hours and only get 1 engram. And if you cheesed a story-boss since this recently-passed Friday to do 789 Draksis (or Gatelord or whatever) kills one-after-the-other-after-the-other then they SPECIFICALLY changed the coin to not work as well during those things.