Not just you mate - I've been running the iOS 9 beta for a few months now and it hasn't been adding anything.
My first experience with this was when I checked in 2 days in advance of an up-and-back trip to Canberra, and thought that seeing as I checked in for the whole return trip at the same time as the outbound flight, maybe it had something to do with that. I provided the feedback to VA on the website, just saying "hey, running iOS 9, didn't have the add to passbook/wallet icon anymore and the OS isn't recognising the boarding pass to add to Passbook (like it used to before it had the dedicated Passbook button)". Basically got a "thanks for the feedback and sorry it didn't work for you" response.
Happened again on my MEL-BNE-CNS and CNS-SYD-MEL flights.
I would hazard a guess that something's changed in the coding in the OS, possibly even just renaming Passbook to Wallet is enough to throw it, if the website is specifically looking for an app called "Passbook".
Totally agree though, it's frustrating, but I suppose all we can do is continually give the feedback that the Add to Wallet function has been lost and hopefully they might get it on to their web team to look at correcting. But definitely not just you.