1) Ensure that the 'Edit custom title' is set for 'Allow'.
2) Ensure that no other group has 'Edit custom title' set for 'Never'.
3) Check your XenForo AdminCP options: AdminCP > Options > User Options > User Banners > Hide Standard User Title - Check these settings to be sure they are appropriate for your desired function.
4) Lastly, ensure that you don't have any Addon installed that may have control over the user title function.
I would at least start with this list and then see if you can solve the problem. If not, post here again. Good luck.