Generally, this happens when the guest has already been rewarded with the coupon code from the mass campaign in the past. If the coupon code uses per guest is configured to 1 in the coupon campaign, then when sending the test email via the mass campaign, the guest will receive the same coupon code with the expiry old expiry date as only one coupon is awarded to the guest.
If further assistance is required, submit a ticket to Punchh Support. (For help submitting a ticket, click here)