How do I retrieve old text messages off my iphone for a particular contact that isn't retrieving as far back as for other contacts/numbers

Hi Landershute,

Thanks for coming to Apple Support Communities for help today.

There wouldn't be a way to retrieve these texts unless you located an old backup from a computer that you could restore. You can find the steps here: Restore your iPhone, iPad, or iPod touch from a backup

"Restore your device from a backup on your computer

1. On a Mac with macOS Catalina or later, open the Finder. On a Mac with macOS Mojave or earlier, or on a PC, open iTunes.
2. Connect your device to your computer with a USB cable. If a message asks for your device passcode or to Trust This Computer, follow the onscreen steps. Get help if you forgot your passcode.
3. Select your iPhone, iPad, or iPod touch when it appears in the Finder window or iTunes. Learn what to do if your device doesn't appear.
4. Select Restore Backup. Learn what to do if a message says that your device's software is too old. 
5. Look at the date of each backup and pick the most relevant.
6. Click Restore and wait for the restore to finish. If asked, enter the password for your encrypted backup.
7. Keep your device connected after it restarts and wait for it to sync with your computer. You can disconnect after the sync finishes."

If you did manage to locate a backup, keep in mind restoring it would erase data saved on your iPhone since that backup.

Have a fantastic day!