snapshot unresponsive? check for shortcut changes:

3.4: web.cyb/mac/snapshot unresponsive? check for shortcut changes:
. the mac snapshot shortcut no longer works;
one reason could be Apple had the bright idea of
moving the shortcut to Shift-Command-5:
(macOS 10.14 Mojave was released on September 24, 2018)
In earlier macOS versions than Mojave,
use Shift-Command-3 or Shift-Command-4 for screenshots.

Cedric Grantham suggests restarts help.
also check in system pref's/Keyboard/Shortcuts,
that the shortcut is still active and not modified.

why use cmd-shift-number instead of just cmd-number?
here is what command-number points at:
Command-1: View the items in the Finder window as icons.
Command-2: View the items in a Finder window as a list.
Command-3: View the items in a Finder window in columns.
Command-4: View the items in a Finder window with Cover Flow.

create your own shortcut?:
Apple menu/System Preferences/Keyboard/Shortcuts.
....
You can create keyboard shortcuts only for
existing menu commands.
If you create a keyboard shortcut that already exists
for another command or another app,
your new shortcut won’t work
—you need to change your new shortcut or the other shortcut.

Apple`keyboard shortcut doesn’t work on Mac:
It’s possible to have keyboard shortcuts
for one app that conflict with another app.
For example, Command-Space bar opens the Spotlight menu,
but in another app it might be [for other]
. in Apple menu/System Prefs/Keyboard/Shortcuts,
A yellow warning badge appears next to conflicting keyboard shortcuts.
Click the shortcut, then change it.