Pass in wallet is OK on iPhone, but shows as expired on Apple Watch

I have a pass (for my son's sports club) in the Wallet app on my iPhone and Apple Watch - on the iPhone it works OK. However, on the Watch it is showing as expired. Predictably Apple (after much troubleshooting) say it must be a problem with the pass, but the club say the pass must be OK as it is working on my iPhone. Has anyone got any ideas on what could be causing this to happen, and how it could be fixed? I have tried removing and re-adding the pass, many times, with no improvement on the situation. The pass does not have the "valid to" field populated, only a a "valid from". As far as I can remember it did work on my old Apple Watch, but as that was a non-cellular version I usually had my iPhone with me, now that I have a cellular Apple Watch I occasionally go out without my iPhone, hence I noticed this issue.