Why do some of my invoices say they have been sent but they actually do not send and I know the email is correct and customer still doesn't receive it

Welcome to the Community, @steve-chris-mass.

There are some possible reasons why your customers aren't receiving invoices sent through QuickBooks. I'd be glad to share them with you and help you from there.

Here are the possible reasons:

• The emails might have been marked as spam. 
• Many email providers, such as Hotmail, use various blocking mechanisms on their mail servers. Your customers' emails were blocked by their email provider. 

Firstly, let your customers check their Spam or Junk Mail folders to verify if the invoices were moved there. If they aren't seeing it from there, let's try clearing and re-entering your email address in the settings to refresh the system.

Here's how:

1. In your QuickBooks Online, click the Gear icon and select Account and Settings.
2. Go to the Company tab, then click the pencil icon in the Contact Info section to edit.
3. Remove the email address, then re-enter it again.
4. Hit Save, then Done.

I'm adding this article for more information: 3 solutions for when customers aren't receiving your emails.

Just in case you want to email and customise sales form, feel free to check out this article for the detailed steps and information: Email and customise sales form.

Keep in touch if you need any more assistance with this, or there's something else I can do for you. I've got your back. Have a good day and keep safe.