If they're like many monthly magazines, the monthly publications may be sent by the print house or a magazine distribution house. So they (CQ) may have to wait just like their customers to receive the monthly publication. If a bunch of sent magazines for a particular month, got lost, or misdirected, in the mail (hard to believe, it's the USPS), and CQ sent their stock to cover the loss, depending on the quantity of loss, it's conceivable their stock for a particular month could go to zero.
Pete, wa2cwa