If you used honey or simple sugars, that could be the cause. Sugars are more fermentable than malt sugars, as honey and/or sugar in water will ferment often to .990.
If you didn't use any simple sugar, then I don't know. I have had one beer stop at 1.007, by mashing very low for a more fermentable wort, and it was clean and crisp tasting.
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