Our plugin can only sync orders that are placed after the Order Syncing Setting is enabled. Previous orders that were purchased while the Order syncing was disabled will not be synced.
On the case that your orders still don't show even when it was placed after activating your Order Syncing, do check on the 'Order Status' of your settings to make sure your order syncing is configured correctly. If the order is still not syncing make sure that the products in the orders in in QuickBooks Online. See Order Syncing Setting.