Messages thread missing

my messages app on my imac is only showing messages for one particular number right back up to December 2023, however a few months ago, I could go back to 2021, manually. However when I search for words in conversations it will bring up conversations for this said number prior to December 23, which is great as it shows they’re still there but how do I get it to show me the entire messages thread so that I can go through message by message as far back as I need to go. Very important that I retrieve these.