Whenever Sanity OS can't map leads in from your web form it will send them to the notes box.
Be sure you've done the following correctly:
1. Click Database manager, click options on the database in question and then click Add/edit webforms. Now click MAP FORM. Ensure that the webform entries (particularly for drop downs) have the same options as the lead form (including spelling and case, although case isn't too important).
2. If you're using API ensure that your website is posting to the correct url with the correctly mapped information as configured in point 1 above.
3. Test your webform by clicking on Database manager, options on the database in question and then clicking add/edit webforms. Now click API at the bottom of the page you will see a form which mimics your API, you can use it to test entries. When you do they will appear in your inbound lead section.