Answer: Concord grapes should be sweet when ripe so I suspect you're harvesting yours before they're fully mature. Or, it's possible your grape vines are attempting to ripen too many clusters at one time, with little success. Try thinning the grape clusters out when they first start to develop, leaving only 3-4 per vine. Thinning will divert the plant's energy into ripening fewer (but better tasting) grapes.
Hope next season's harvest is sweet and juicy!
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