Classic and Agenda Calendars have different registration steps
It's possible one of the reasons some people don't get a registration confirmation email is because they didn't actually complete the steps. That's what happened to me a few days ago. I just went through both types again in the last 3 days to confirm my impressions.
If looking at rides through "Classic" calendar and clicking "Register" gets you registered. ONE click.
If you are looking at the "Agenda View" calendar, you have to do THREE clicks to register. It's strange, because at the second click, there seems to be an option to "add more registrations", which is present after successfully registering via the Classic calendar. That's what threw me off the 1st time, convincing myself that I had registered.
Turns out I had to click "Next" multiple times to actually finish.
If the buttons were more obvious, perhaps bold RED, or some other eye catching method...
Anyway, I don't know if it's possible to follow up on people who have had missing emails, etc.