A little known fact about the Knowify/QBO sync is that a specific tax code needs to exist in your QBO database for these two programs to talk to each other with Customer Invoices.
If you try to sync, and the invoices don't appear in QBO, check if you have a "Zero-rated" tax code in your database. If you converted over from a Quickbooks Desktop file, you may not. But you need one. And it can't be an E, or a Z, but actually has to be a tax code called "Zero-rated" - formatting and capitalization exactly like that.
Once you've set that up, and you go to sync your invoices, it should work.
And just so you know, it doesn't mean that your invoices will be zero-rated, for some quirky reason, it's just needed in the QBO database.
Learn more about Knowify.