Thanks for adding more details, nctriangle.

The attachment may be missing because it reached the maximum file size limit of 20 MB. I'd suggest reducing the size and attach the document again.

Here's how:

1. When creating an invoice, click on the Show existing link.
2. From the right side, click on the Add link for the document.
3. Then, don't forget to check the boxes of the attachments.

To learn more, please see this article:  Attachments in QuickBooks Online.

If the issue persists, I'd recommend contacting our QuickBooks Support Team. They'll be able to determine the main cause of this.

1. On the Assistant tab, click Talk to a human.
2. Enter your issue or question on the Type something field, then click the send icon.
3. Choose I still need a human.
4. Select Get help from a human.
5. You can either select the Chat with us or Have us call you option.

Additionally, I've added an article that'll help you personalize and add specific info to your invoices: Customize Sales Forms.

Please reach out to me if you'll need assistance in managing sales transactions. I'm always here to help.