Thanks for posting here in the Community, allisonguess.
Most common display issues might be caused by too much cache files in the browser. This is the possible reason your customers are unable to see your invoice sent.
To help fix this, I recommend accessing your QBO account via a private browser. You can also have your customers access the View invoice link through a private browser. It's a great way in isolating the issue since it doesn’t store data in the cache. Here are the following keyboard shortcuts:
Google Chrome: CTRL + Shift + N
Safari: Command + Shift + N
Mozilla Firefox: CTRL + Shift + P
From there, you can create a test invoice and resend it. If it works, you can go to your regular browser and clear its cache. This deletes the stored cache files and get a clean slate in the browser. If not, you can use other supported browsers.