I found that there is no tool bar in the bottom where "Preview, Save, Close, or Publish" buttons are. In this case there is no other way to save the document, though all changed are saved automatically but merely as drafts that could not be published.
It has been occurring quite frequently in recent 2 weeks.
I was wondering if it is caused by the setting in one particular page or a system bug. Thank you in advance~
It should be:

Now:
