There are a number of reasons why Truecaller might not be able to identify the caller ID of an incoming call for you:
1. Your Truecaller app has been force-quit and is not running in the background.
2. You have a slow or no data connection (needs 3G, 4G or strong WiFi).
3. The incoming call is from a non-Truecaller user.
4. Caller ID might not work if the battery level is too low.
5. For Truecaller iPhone callers: The caller didn't initiate the call from inside the Truecaller app (Truecaller's built-in dialer), Widget or Action extension.
If any of the statements above occur then unfortunately Truecaller will not be able to show you live Caller ID.