Command Palette shortcut not working in Sublime Text3

Something that I found:

Weirdly my Sublime Text 3 doesn't recognise the command if I use LCTRL abd LSHIFT. Using RCTRL+RSHIFT+p opens the command palette, so try that.

The two main reasons for this sort of situation (regardless of the key sequence in question) are:

1. A user installed plugin or custom key binding is bound to the same key, which is taking precedence and stopping the action that you expect from happening
2. Some external process is eating the keystroke before Sublime even gets to see it.

In order to diagnose which it might be, you can open the Sublime console with View > Show Console or by pressing Ctrl+`, then enter the following commands:

sublime.log_commands(True)
sublime.log_input(True)

Once you've done that, press the key sequence in question and check the output in the console. In your specific case, you should see this:

key evt: shift+control+p
command: show_overlay {"overlay": "command_palette"}

Not seeing the command that you expect indicates that some other action is bound to the key in question, and usually the command will lead you to what's causing the problem.

Not seeing the key event means that some external process is eating the key. This could be some global program or it could be something in the OS doing it (in the case of Linux, the window manager).

It's also possible that you see a different key event entirely, which indicates that your keyboard layout is not what Sublime expects.

Depending on the situation you may be able to disable whatever is eating the key. Presuming you can't find what that is or otherwise don't want to disable it, or if the event shows that Sublime is seeing a different key, the solution is to change the key binding.

The procedure for this is to find the binding that's not working and copy it to your custom key bindings, changing the key as appropriate to something that Sublime can see.

For core Sublime key bindings, look in Preferences > Key Bindings to find the key in question. For packages, that's generally in Preferences > Package Settings > PACKAGENAME > Key Bindings.

In your case, the setting is a default key binding, so looking in the default key bindings yields the following binding, which you can put in your custom key bindings and change as needed:

{ 
    "keys": ["ctrl+shift+p"], 
    "command": "show_overlay", 
    "args": {"overlay": "command_palette"} 
},

In my case it was an app called https://noiz.io which had stolen this shortcut. It can take a bit of time as there isn't (AFAIK) a way to find the application which is bound to a shortcut.

In my case a Pomodoro app I just installed had a command for starting clock defined with the same shortcut and it was stealing the event from Sublime. Just removed the shortcut assignment and it works now.