Limited search results in Outlook
A common issue is that Outlook has a setting that restricts the number of results shown when you search, while this speeds up searching its effects are for the most part negative for the user. This can appear as "my old emails are missing" or "I know that email is here somewhere"
You need to disable a setting in Outlook that restricts the number of results:
In Outlook, go to File - Options - Search
Then uncheck the "improve search speed..." option and hit OK