I have gone back to the basics to test this. 
- I create a project
- Create a sales order from the project
- Sales order is for $2500, project category is Install; item is a service line item
- Confirm, Pick, Pack the sales order line; shows as Delivered as it should
- Create invoice proposal - include sales orders, leave dates blank
- Result - No invoices were created;
- A few times the invoice is created, but for 0 amount
 
Any ideas?
  
                            
                         
                        
                        
                            
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