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