The troubleshooting steps here are ordered by most-common to least-common cause of issue.
Dig through the settings in the Display to enable all CEC functions.
Contact the manufacturer to confirm that the display supports the desired function; most displays we have tested will respond to Power On, Power Off, and HDMI Selection. Past that, it is very hit-or-miss.
Be sure you are using the correct CEC codes. Built-in and custom codes can be found at the CEC over IP Control article.
Some endpoints seem to be 'lazy', meaning they do not respond to the first command sent after a long idle time. If dealing with a 'lazy' endpoint, send the command back-to-back with a short delay between.