Part 1
Timestamps for Part 1:
00:17: Checking for signs of a broken connection
01:15: Check that the Facebook Form is mapped
03:02: Check the Workflow Trigger for Facebook Lead Form is correct
04:53: Checking the Connection with the Facebook Lead Ads Testing tool
Link to Test: https://developers.facebook.com/tools/lead-ads-testing/
06:17: How to Restore Leads Access from your Business Manager
Link to Business Manager Settings: https://business.facebook.com/settings
If all of the above has not worked, please proceed to Part 2 below where you can reset the Lead Form connection my un-mapping and re-mapping the form.
After this, please test the form again at 04:53 in the first video above, using the link above.
Part 2 - Resetting the Facebook From Mapping
Navigate to the Facebook Form list via Settings > Integrations > Facebook Form Fields Mapping
Find the form you would like to reset and toggle the status button on and off
Select the corresponding green 'Edit Fields' button and review the mapped fields to make sure they are matching
Select the green 'Confirm' button to finish resetting your Facebook Lead Form