I have a problem that follows me from 2015 MacBook Pro with macOS Sierra → macOS High Sierra → macOS Mojave and now on to 2018 MacBook Pro running macOS Mojave.
I have configured two input sources: English and Russian, and I prefer to switch between them with Caps Lock key.
The problem: Some times switching does not work. I press Caps Lock key and the layout stays the same. I press it again and it still stays. Eventually, it gets changed but it takes 5-6 Caps Lock key press.
This happens in both cases, US → RU and RU → US.
I have Siri disabled.
Best Answer
I had the same problem. As I understand it, this is intentional behavior by Apple (they've been known to have additional delay on Caps Lock key to prevent accidental presses). Not many people notice this because it happens only when you type really fast (to reproduce just try spamming one key and press Caps Lock to switch the language at the same time – sometimes it would just be ignored).
There's a way to solve this – use Karabiner Elements. You can find how to set it up here: https://hybridtechcar.com/2018/07/01/turn-caps-lock-into-a-super-button-suitable-for-any-macbook/