Hi Ayannai,
As indexing is a process that occurs in the background of devices and can only be manually triggered on a Mac, the best options are to take general steps for this behavior.
First, ensure that you have the latest version of software on your device. Some software updates will trigger a reindex of files and may clear up the problem you're facing. See Update your iPhone or iPad for options on this.
If you haven't already, restart the iPhone and ensure you have a strong data connection for any processes to complete.
Confirm that you do not have restrictions in place, security software or profiles installed that may interfere with the Mail app or these functions.
If those fail it could be necessary to remove the email address and reinstall it to force those messages to index again.
Regards.