An item can be missing from search results for a number of reasons.
If the item shows up in search for the Administrator, but not for the user, this is a permissions issue. Search will not display items the user does not have view permission to.
Check that the user has permission to the site, application, and folder.
Search Exclusion Settings
At a site level, items can be excluded from search.
At an application level, items can be excluded from search.
At an application level, archived items only can be excluded from search.
Make sure to check all three options to see if the administrator has purposefully excluded the item from search.
Error During Search Update
Items in search are immediately added to the search index when they are added or updated. Deleted items are immediately removed. To test this, edit the item and see if it shows up in search.
Alternatively, if multiple items are missing from search, you can re-index the entire intranet.
Note: while the intranet is re-indexing, the intranet will still be available, but performance will be significantly degraded. Also, the first step in the re-index to to erase all search records. As a result, search will not return matches until items have been re-added. This should only be done off hours.