Occasionally my 13" won't shut-down

Sometimes when I try to shut-down my 2012 13" MacBook Air, it just doesn't.

I leave it for five minutes, and when I come back it still hasn't shut-down.

So I open an app, for example Safari, and it displays a dialog window saying "Shut-down was interrupted by the Application 'Safari'.

I try again, and still the same outcome.

The only way I can get it to finally turn off is either if I hold down the power button (which isn't recommended on a regular basis) or keep using my Mac until finallt it decides to shut down, which can take up to hours.

Is anyone else experiencing the same? How can I fix this?