Tickets in Apple Wallet will not scan.

Hi. Several times now - tickets that I have downloaded to my wallet will not scan. They add and show up in my wallet fine. But then day of event, at the entrance the venue is not able to get my ticket to scan. It has happened with ticketmaster ticket and a sports ticket (so two different companies and venues). For ticketmaster, the one in my wallet didn't work - but them when I pulled it up within the ticketmaster app - that worked, so it does not seem to be an issue with the screen of my phone. Help???