I just recently started having this problem with Oakland Athletics tickets. After spending time on the phone with ticket services, the only solution we found is to have them resend the tickets in an email, which gives the option to Add to Passbok in the email. According to the rep I spoke with, they are the only ones who can send this email for the tickets in my account. I am a season ticket holder and so I don't get emails to print or add to passbook for any of the tickets.
None of the other solutions we tried worked. We tried logging in through the At the Ballpark app, logging in via Safari, and tried scanning the barcode of the printed tickets from the Passbook app. When trying to add the tickets to passbook directly from the My Ticket Inventory screen of the mobile web site, it always says Download Error - Safari cannot download this type of file.