Hi dennisfrombc,
Welcome to the Apple Community.
If you haven't already:
✽. You might check your eMail account(s) online to see if there are any settings for how long new messages are kept,
or, if there is a limit on how many eMails (or Mb) are stored. (You may find the older eMails in the inbox(es) there ?)
✽. You might want to double-check what preferences are set in Mail app on Mac:
Change preferences in Mail on Mac – Apple Support
✽. You might want to try searching for a particular older eMail
(Perhaps they are somehow being "archived" in another mailbox?)
Search for emails in Mail on Mac – Apple Support
or
Show emails from VIP senders in Mail on Mac – Apple Support
✽. Might there be unwanted filters or rules set ?
Eg: You can use filters to show only certain email messages in a mailbox
Filter emails in Mail on Mac – Apple Support
Eg: A rule could be set to automatically move messages from your inbox
Use rules to manage emails you receive in Mail on Mac – Apple Support
All the best :-)