Hi Sue

This sounds like the issue fixed in the February 2020 release TL-23647 

Fixed 'Declare interest' functionality when a user is booked onto a past event

                   Previously a "When no upcoming events are available" option is enabled for
                   Seminar, the "Declare interest" functionality worked for no upcoming events
                   and no past events if a user is booked onto a past event. Now it is fixed
                   and the user can declare interest if there are no upcoming events and the
                   user booked onto past events.

Upgrading to this release should fix this.

Regards