You may have duplicate customers when you convert a lead. This occurs when:
If you are in QuickBooks Desktop and try to convert the lead to a customer, the customer goes into a sync conflict. If you are in QuickBooks Online, you can't even convert the lead because it will create a duplicate customer.
To solve this problem, you will need to have QuickBooks merge the two customers.
To merge customers in QuickBooks, simply rename the duplicate customer, e.g. "Bob Crenshaw2" to the original name. QuickBooks will prompt you to merge them.
Once you do that and it syncs, then all the transactions and all the activities, opportunities, etc., will sync with Method.