An issue (or a separately checked-in copy of the issue) has been skipped and must be claimed.
Go to Trello and find the Claims board, which is part of the pSerials team. Create a card in the Skipped Issues column, with the card number and the journal title as the card's title. In the card, add a comment containing the skipped issue's enumeration and cover date.
[Note that you only need to do this if you desire to wait some days before claiming the issue to see if the issue arrives in the next few days. Some titles, such as Mergent Dividend Record, arrive mostly in order but occasionally there will be a previous issue that arrives a day or two later. On the other hand, for some titles like L’Express, it is better to claim the issue immediately due to narrow claiming restrictions and limited stock kept by the publisher.]