Customer receives statement every month but never seems to get the invoice (although it shows as sent with the time/date on the system)? Any ideas?

Welcome and thanks for posting in the Community, @sarahdee81.

This usually happens when their spam settings are set to automatically filter. You can have them check their junk mail and spam folder. If the invoices are still missing, let's try to clear and re-enter your email address in the company settings.

Here's how:

1. In QuickBooks Online (QBO), go the Gear icon at the top right to get to the Account and Settings. 
2. Proceed to the Company tab at the left pane, then select Pencil (edit) icon in the Contact Info section. 
3. Clear the Company email address field and enter the desired address, even if it shows as the correct one. Make sure there's no extra characters or spaces before, within, or after the address you enter.
4. Hit Save and Done to complete.

After that, try to send a test invoice to yourself to double-check. If the issue persists, I highly suggest following the troubleshooting steps outlined in this article: 3 solutions for when customers aren't receiving your emails.

Please let me know how it goes or if you have any other issues in the comment section below. I want to make sure everything is taken care of for you. I'm always here to help. Have a good one!