There are a couple of possibilities.
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The Ancient One didn't make use of the Time Stone in the same way as Doctor Strange. Whether she couldn't or wouldn't is open to interpretation, but Strange's particular use of the stone did give him the ability to see past his own death.
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Perhaps one needs to be alive in a given time in order to see into it, and there were no futures after The Ancient One's death in which she returned to life, whereas for Strange there were. She couldn't see that future because she wasn't alive in it. This does raise the possibility that there were other futures than the six million he mentioned that resulted in Thanos' defeat... but that Strange couldn't see those because he wasn't alive in any of them.