Fluid Class Search Incremental Index

During registration last fall, we had reports of students not being able to find classes in Manage Classes > Class Search. Now that students are registering for spring classes, we are again getting reports of students not being able to search for existing classes. We now have the additional observation that the classes not being found have start dates after the start of the semester (with a unique session code other than REG). Does the Class Search Index Manager that runs the incremental index not find classes starting after the semester start date? What do we need to do to maintain the class search