Hi, @Silsal. 

I understand how important it is to be notified of our transactions properly. 

When you successfully sent an invoice, you'll be able to see it on your end. Since your customers have successfully received the invoices, but you encountered a delivery issue, we have to perform some basic troubleshooting steps to identify if this is a browser-related issue. 

Your browser's cache saves files on your computer to enable the website to download faster on your next visit. However, it can also get outdated over time, leading to problems in accessing websites. To get started, we have to open your QBO account on an incognito window. An Incognito window will prevent the browser history from being saved. 

Please refer to the following keyboard shortcuts based on your browser type:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

If the private browsing session works, you'll need to clear the browser's cache so the system can restart. This task is also possible in other supported, up-to-date browsers. Clearing the cache will reboot the system, allowing you to work with clean slate. 

Additionally, you can refer to this article in processing invoice payments: Record invoice payments in QuickBooks Online. 

I'll patiently wait for your response. Take care always!