Correct UX for the scenario when a user forgets the answers to their security questions?

We have an application where a user is required upon registration to set up three security questions and answers

When they login to the application, they must answer one of the questions after entering their username and password. Should they forget their password, 2 of the security questions are presented to be answered along with 2 items of data for them to enter (DOB, NI Number etc etc)

There was recently a scenario whereby a user forgot the answers to their security questions(!) and we had to re-register them from scratch which was quite cumbersome.

What is the correct UX for the scenario when a user forgets the answers to their security questions?