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 go to your "Phone settings - > Battery > Details > Find Truecaller > Turn off optimization.
Note, live caller id requires 3G or WiFi to function. This is due to technical reasons.