Indexes make locating records easier, but there are limitations to them. Handwriting can be difficult to read and indexers make mistakes. Indexers may only index key names in a document, leaving out the names of others who are mentioned in ways that can provide significant information on them.
If you have reason to believe a person should be in a record and they are not in the index, search the records manually. Learn how the records are organized to create more effective manual search strategies.
If there is a person you have not found in a record and you have tried alternate spellings and names, review ways in which the records can be manually searched. Sometimes a manual search is more practical than others and not all records can be manually searched.