User searching fails on Full Name but not email

We’ve got around 3000 users on our platform and recently I’ve noticed that searching for a user to message fails sometimes. The fails tends to happen when you are searching with their Name ( Firstname Surname) but if you search their username (firstname.surname) or their email address it finds them just fine.
Any suggestions why this is happening & what can be done to stop it?

Is this specific to any client? Web, Android or iOS?

Good point - I didn’t say!
This is on the web client on Chrome. We don’t have mobile devices using this yet.