Custom keyboard shortcuts not work

1) Description: Ubuntu 16.04.1 LTS
   Release: 16.04

2) ubuntu-settings:
   Installed: 15.10.8
   Candidate: 15.10.8
   Version table:
   *** 15.10.8 500
               500 http://it.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
               500 http://it.archive.ubuntu.com/ubuntu xenial/main i386 Packages
               100 /var/lib/dpkg/status

3) I made some custom keyboard shortcuts under All Settings > Keyboard > Shortcuts
   In particular, I made these:
   CTRL+SHIFT+S - Custom screenshot - gnome-screenshot -f "$HOME/Pictures/Screenshots/$(date +%F_%H:%M:%S).png"
   CTRL+SHIFT+A - Custom screenshot area - gnome-screenshot -a -f "$HOME/Pictures/Screenshots/$(date +%F_%H:%M:%S).png"
   CTRL+SHIFT+W - Custom screenshot window - gnome-screenshot -w -f "$HOME/Pictures/Screenshots/$(date +%F_%H:%M:%S).png"

   I expected the shortcuts working when I press the right keys combination.

4) Custom keyboard shortcuts not work.

   The shortcuts worked only one time and after never worked. If I use the commands directly in a terminal window, the commands work fine.