To improve sync times, fulfilled transactions, like a paid invoice, will not be synced from QuickBooks Desktop if it's outside a date range which you can set in your sync preferences.
This does not mean the invoice doesn't exist. It still is retained in QuickBooks, however, it is not synced over. For most companies, past transactions are not needed in day to day operations.
If you require to have your past transactions in Method, you can extend this range under Transaction History.
Change Transaction History for All Transactions
Click the Profile icon in the upper right and click Integrations.
Select QuickBooks Sync.
Click Expand on Advanced option.
Expand the dropdown for Maintain transaction history on Method for the past and select a time period.
Change Transaction History for One or More Transactions
Follow steps 1 to 3 above to get to your Integrations → QuickBooks Sync → Advanced preferences.
Select Edit history settings per transaction type:
This will open up a new page called History Per Transaction Type. Select a date range for each transaction you wish to keep the history for.