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.