Hi @Charlie G ,
Welcome to our forum!
Personal Contacts from the personal address book will not populate when sending a new email.
Does this issue occur with all personal Contacts or some specific emails addresses?
And when you enter other email addresses, will there be suggested recipients?
If there aren't any suggested recipient prompts, please first check and confirm the "Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines" option is enabled via File>Options>Mail>Send messages:

If the issue only occurs with some specific contact addresses, are you entering them for the first time? If so, based on my test, i found that these addresses are not saved to the autocomplete list until after the first entry. So you need to enter it manually once, and then you will be prompted with the full address when you enter it.
If you did enter it manually but still don't receive a complete address suggestion, i'm afraid that it may be affected that Outlook limits the number of entries that you can save in the AutoComplete list. After you reach this limit, Outlook uses an internal algorithm to determine the best names to remove from the list. Please refer to the following article and try the suggestions:
Limited number of entries

Hope the above help!
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