Difficulties Searching Archive

User archives go back to 2005. When Client program (ver 18.0.1) accesses the user archives, every message is present. Not a single archived email is missing. ... HOWEVER, and it is a big aggravation, when searching the archives the searches sometimes fail to find any messages under search terms that are absolutely present and accessible without using search. Other times the searches find 10% of the messages searched for. Seemingly covering only a certain time period, notwithstanding that no restriction is entered on a search

What is going on? -- Live mailbox data searches work fine.