For the same reason: each prize's Coupon rules names a coupon by its number, and that coupon does not exist, or is a different one, on the other site. A prize that had coupon rules there arrives marked Needs coupon rules and cannot be won until you choose its rules on the Wheel screen, or untick the mark to use the Coupon Defaults. The message after the import says so. Prizes that had no coupon rules arrive without any.
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