Cannot move or drag fields when customizing an opportunity form

1. Navigate to Customization > Forms > Transaction Forms > Click Return to Upgrade Checklist
2. Make sure the preferred Opportunity Form is deployed
3. Edit the preferred Opportunity Form > Screen Fields > Notice that fields after the Detail field cannot be moved within the form unlike those fields above it.
 
Solution:
 
1. Navigate to Customization > Forms > Transaction Forms > Edit the Opportunity Form
2. Under Screen Fields, Look for Details
3. Set the Field Grouping as blank
4. Save the form
5. Go back and edit the form
6. Notice that the Details now appear as the last on the list, leave it as it is until finished in re-arranging the fields
7. Once done, set the Field Grouping back to Primary Information
8. Save