After upgrading my machine to macOS Sierra 10.12 I noticed an issue using the keyboard: when holding a random key the cursor on the screen was not moving.
It seems that some keyboard settings suddenly changed after the upgrade. After searching for a while I noticed that the ‘Key Repeat’ slider in the Keyboard settings was set to ‘off’.
I have no idea why this was introduced in Sierra. A bug maybe? Regardless, moving the slider to the right did fix my problem. If you’re also having this issue I hope this post can help you :)