Many local records are unindexed. Often these materials are organized chronologically, by the date of the event. There may be indexes, but they may cover a small time frame, perhaps one index per year or an index for every ten years. In these cases, it will be necessary to estimate the date of the event--birth, marriage, death, etc.
Before searching these records, use other materials to estimate the date of the event. It may be possible to narrow down the range of years before a manual search is conducted. And remember, that not every set of records has an index...and even though new indexes are being created to some records, it may not be for the ones you need.