Old messages have disappeared on iPhone that I haven’t deleted?

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I have gone into my history to retrieve an old text message but there aren’t any there to that person. Any idea how to get them back or why they’ve deleted?

Tia

Go to Setting/Messages and scroll down to Message History, which controls how long messages stay on the phone - mine is set to Forever, as my phone has plenty of room, but it can also be set to 30 days or 1 year.