There could be several reasons why your Mac may not be detecting a second monitor. Here are some steps you can take to troubleshoot the issue:
1. Check the connectivity: Ensure that both ends of the video cable are securely connected to both the Mac and the second monitor. Verify that the cable you're using is compatible with your Mac's video output ports, such as HDMI, DisplayPort, or Thunderbolt.
2. Verify power and display settings: Make sure the second monitor is powered on and set to the correct input source. On your Mac, go to "System Preferences" and then "Displays". Check that the second monitor is recognized and set up properly.
3. Adjust display settings: Within the "Displays" settings, click on the "Arrangement" tab and make sure the "Mirror Displays" option is unchecked. Adjust the arrangement of the displays by dragging the white bar (representing the menu bar) to the desired monitor. Sometimes, the second monitor is detected, but the display may not be extended to it.
4. Restart your Mac: Sometimes, a simple restart can help resolve connectivity issues. Restart your Mac and see if it detects the second monitor upon reboot.
5. Update macOS and drivers: Ensure that your Mac is running the latest version of macOS available. Go to "System Preferences" and click on "Software Update" to check for any available updates. Additionally, check the official website of your monitor's manufacturer for any driver updates that may be necessary for compatibility.
6. Check for hardware or cable issues: If possible, test the second monitor with another device to see if it works properly. If it does, the issue may lie with your Mac or the cable being used. Try using a different cable or connecting the second monitor to another Mac to identify if the problem is with the Mac itself.
7. Reset NVRAM or SMC: In rare cases, resetting the NVRAM (non-volatile random-access memory) or SMC (system management controller) on your Mac can help resolve display-related issues. Instructions on how to perform these resets can be found on Apple's support website or through a quick internet search specific to your Mac model.
Remember to follow these troubleshooting steps in the given order to systematically identify and potentially resolve the issue of your Mac not detecting a second monitor.