Did you click "File-Save Oekaki" from the top bar? After the loading bar is finished it should ask you if you want to, A: Save the drawing and finish later, B: Continue Drawing, or C: Post it now.
If you have tried this, maybe try saving it by clicking the "Save drawing" button next to the "Undo" and "Redo buttons.
If you have tried this, maybe try saving it by clicking the "Save drawing" button next to the "Undo" and "Redo buttons.