1-For Invoice/Bills having multiple line items then the loopover should be
CRM to QB Sync process – Target(QB Destop and QB Online)
(Target Table- Invoice) list/invoicedetail
QB to CRM Sync process – Target(Quickbooks desktop) Target(QB Online)
(Target Table- Invoice detail) Invoice Ret/InvoiceLineRet Line/SalesItemLineDetail
2-Max Batch size that can be kept is 200 in reader and in writer it is advisable to keep as 10. So,in this way the reader will make a batch of 200 invoices and writer will sync in batches of 10 till all 200 invoices/bills are synced.
3-Error which you were receiving due to which invoices were not syncing-
Please make sure you are adding invoice date for this year when running the sync process for that record.This will resolve this issue