Samsung recently introduced a new Smart Manager feature that helps you manage your battery time. This is a great new feature but if enabled for Truecaller it could mean that you could disable caller identification and spam blocking by mistake.
To ensure that you’ll always be notified about who’s calling do the following:
1.) Navigate to your phone settings
2.) Tap on Battery
3.) Tap on "Details" or "Info" > Find Truecaller > Turn off optimization.
Note, this is required for blocking calls as well.