Make sure you have connected a Quickbooks Online account
Go to settings and scroll down to the section shown below to verify it says "connected"
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Make sure you have the "sync invoices payments and refunds" toggle switch activated
In the Quickbooks Online settings section, verify the "Sync invoices, payments and refunds toggle is turned on as indicated below
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Make sure at least one line item exists on the invoice
Invoices without any line items will not be synced

Make sure you haven't deleted or made the customer inactive in Quickbooks Online
If the customer exists in PB and you have deleted or made it inactive in QBO, invoices and payments (or anything else) will not sync, even if you create a new customer with the same name.

Make sure you haven't deleted the payment or invoice in Quickbooks online
If the invoice or payment previously synced but you deleted it in QBO, the remaining invoice or payment in PB will no longer sync to QBO. You would need to delete the payment or invoice in PB and then re-create it in PB in order for it to sync to QBO again.

Make sure "Automatically apply credits" and "Automatically apply bill payments" settings are turned OFF in Quickbooks Online
These settings will interfere with the syncing process and must be turned off. You can have payments apply automatically to newly auto-created invoices directly in Pool Brain.

Make sure you haven't activated the "Shipping Charge" invoice field in Quickbooks Online
"Shipping Charge" is not a common field to have activated and we do not sync it. Therefore, if it is activated on an invoice in QBO, it will prevent the invoice from syncing to PB.

If an invoice/payment exists in Quickbooks Online but not in Pool Brain, verify that the date of the invoice/payment isn't older than the date you activated the "sync invoices, payments and refunds" toggle in Pool Brain settings
By design, nothing that existed in QBO prior to the date you turned on the sync toggle will sync over to PB on it's own. However, anything new that is created in QBO will sync to PB and anything old that is EDITED in QBO after you turn the sync toggle on will sync to PB. So if you want something older to sync into PB, all you have to do is edit anything on the payment or invoice in QBO. For example, you could add any text in the "memo" field for the invoice or payment in QBO or

alter anything else you'd like.

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If an invoice exists in Quickbooks Online, but has a payment applied to it that is older than the date you activated the sync toggle, the invoice will not sync to Pool Brain
You can un-apply and re-apply the payment to the invoice in QBO and then both the payment and invoice will sync to PB.