not all search results appearing

When doing a search of the sent items folder, doing a search for the name of a recipient does not always give all of the results. Going thru the sent items folder manually clearly shows the messages are there but they do not come up when searched. Many of the suggested troubleshooting steps for spotlight search issues have already been tried with no luck (uninstalled/reinstalled office, deleted and re-added the exchange account, and re-indexed the OSX spotlight search multiple times). The issue seems to happen randomly as sometimes the search will work and in other cases it will not. Any other suggestions for troubleshooting would be  greatly appreciated.