when you set variable or message using ->with() it doesn't set the variable/message in the session flash, rather it creates an variable which is available in your view, so in your case just use $success instead of Session::get('success')
And in case you want to set the message in the session flash the use this Session::flash('key', 'value'); but remember with session flash the data is available only for next request.
Otherwise you can use Session::put('key', 'value'); to store in session
for more info read here