Some guests have joined my event, but now I am not able to see them any more in the Guests tab on my Event Profile. What might be the reason?

You might not be able to see guests in the Guests tab, as you may have edited the occurrences of your event.

When you edit the occurrences of your event, then new occurrences are created and guests associated with the previous occurrences automatically leave them.

You will see guests in your event after members join new occurrences of your event.