Your wording is not clear, is it not showing up in either the backend or frontend? Or is it showing on the frontend, but not in the backend?
Are you getting a confirmation email of the signup?
If the signup is not showing on both the frontend and backend, then you may want to check your server error logs to see if there are any server errors at the time of the signup.
If it’s only on the backend that it is not showing, it could be a browser caching thing, and you can hold shift and reload the page a few times to try to clear the browser cache. If the date has already passed, then it won’t show on the backend unless you make sure to check the checkbox for Show Expired Tasks? in the main settings, and also make sure the checkbox that says Automatically clear expired signups? is NOT checked (it will delete signups automatically when the date is past to keep your database size under control).