I have been using companion certs for many years now without any issues, but since February 2023, I have not been able to find any First class (even with multiple Z availability on ALL segments) to show up on DL website. Only main cabin fare is shown.
This seems to happen when there is a connection involved. If I search for a routing that offers direct flights, I do get First class offering. I don't know ... is going on and what restriction DL is applying now. But living in a non-hub city, most of the flights involve a connection to where I want to get to, and is rendering my cert useless.