Welcome to the group, there’s plenty of other Zwifter with pretty much the same issue.
There are thread with hundreds of posts both here as well as in the “bugs and support” forum or how it is called.
You can find some also on Zwift Power forums.
Some users say that depends on connectivity. Other says that it often happens in large events. I have opened a support ticket to Zwift one week ago without answer yet.
Depending on the level of frustration you can get if your event doesn’t get recognized, I suggest you either keep joining and hope, or stop till when it will be more clear why this happens and how to avoid it.