What were the original expiration dates of the tickets/credits? It is possible that Delta is tightening up the rules on these. Technically, a ticket has a shelf life of one year from the ORIGINAL ticket date. (All travel must be completed one year from that date). Reissues should not extend that date. Their system has been generous the last few years. If they start to enforce that policy and follow through on the rules, there will be a lot of surprised ticket holders.