If a key combination doesn't work at startup, one of these these solutions might help:

Be sure to press and hold all keys in the combination together, not one at a time.

Shut down your Mac. Then press the power button to turn on your Mac. Then press and hold the keys as your Mac starts up.

Wait a few seconds before pressing the keys, to give your Mac more time to recognize the keyboard as it starts up. Some keyboards have a light that flashes briefly at startup, indicating that the keyboard is recognized and ready for use.

If you're using a wireless keyboard, plug it into your Mac, if possible. Or use your built-in keyboard or a wired keyboard. If you're using a keyboard made for a PC, such as a keyboard with a Windows logo, try a keyboard made for Mac.

If you're using Boot Camp to start up from Microsoft Windows, set Startup Disk preferences to start up from macOS instead. Then shut down or restart and try again.

Remember that some key combinations are disabled when your Mac is using a firmware password.