You may have an issue in which a user or group of users see an error message for password enrollment that states,
"The number of questions required for enrollment has changed. Enroll again."
The number of users affected by this is unknown until a report is run. There is a built in report on your instance named "Password Reset Enrollment By Verification".
You can view the report by the following methods.
Depending on the verification method that you are using, each instance may have different results shown for the report.