I just did a test and found that the on form submit event it's not being triggered when a response is edited.

I'm not sure if the on form submit trigger is working as intended, if the above is due to a bug or to a glitch. To be sure, post an issue to the [Google Apps Script Issue Tracker](https://code.google.com/p/google-apps-script-issues/issues/list).

As a workaround, instead of using the on form submit event, use another way to run your script, like a time-drive trigger.

# References

- [Custom menus in Google Apps - Google Apps Script Guides](https://developers.google.com/apps-script/guides/menus)
- [Simple or installable triggers - Google Apps Script Guides](https://developers.google.com/apps-script/guides/triggers/) 
- [Google Apps Script Support](https://developers.google.com/apps-script/support)