I had the same problem with the transfer and hotel vouchers for my trip to New York next month. Using the 'Print non flight vouchers' link towards the bottom of the MMB screen didn't work..

However, I found that if I went into MMB, scrolled down only to where my flights are listed, below the flights it says 'hotels'. Clicking on that opens the hotel details and at the bottom of that it says:

Print non flight vouchers

Clicking on that link then opened up the full list of vouchers to print off (including the transfer vouchers) and all printed off okay.