Command bound to a Checkbox (in DataGrid) does not fire

I have a MVVM DataGrid, and one of its columns is a CheckBox. The CheckBox is bound to a Command.

XAML

I want that each check/uncheck will fire that Command. Unfortunatly, the Command does not fired. As an experiment I added a Button which is bound to the same command, and when I click the button – the Command is fired! What is wrong with the CheckBox?

Thanks