Hello Headphanie,
We have a great support article that might help with this. Please take a look at it here: Search in Messages on iPhone - Apple Support. Please start out by making sure these are the steps you are following for searching your Messages.
If those are the steps you are trying, but the issue persists, further assistance may be needed. We want to make sure you reach the right team for help with your Messages search issue, so we recommend that you contact Apple Support directly. Depending on your region, you may be able to reach them here: Get Support - Apple.
Thank you for participating in the Apple Support Communities and have a wonderful day.