Google Colab IPython notebook files often fail to be saved
You can check disk space or permits, sometimes drive and collab became unstable.
Also check if it can be saved while kernel is free and not busy.
I'm getting a terrible trouble with my Deep Learning porjects. My google colab files mostly fail to save. The status shows

Saving Changes...

However, never succeeds. After a while

Automatic document saving has been pending for n minutes. Reloading
may fix the problem. Save and reload the page.

Reloading is not the remedy and after reloading, the problem is not solved. I really don't know what to do with it. Any ideas?