The problem is not refaring "in quick succession." The problem is refaring. I was told by a CSR that the voucher mailing is outsourced, even though the return address says "Chicago" on it. IME, 90% of my vouchers never arrive on the first try. Other people have excellent success rates. Nobody knows why some vouchers never arrive and others do.

To get vouchers reissued, you have to call Refunds. Provide them with the ticket number and/or voucher number (whichever you have), explain that the voucher never arrived, and ask them to reissue it. Ask them to keep you on hold while they reissue the voucher - it doesn't give you any guarantees but it makes it more likely that they will handle the reissue request right then and there, rather than putting it on a "to do" list and then forgetting about it after handling a few more calls.

But, in short, the problem is simply that the voucher mailing system is fubar'd, for some unknown reason. However... you do have to give them 4-6 weeks from request to complaint, as it may actually take that long for the vouchers to arrive.