I'd like to know more about your concern, Catherine19. What transaction are you trying to create?

This will help me check our records if there are similar cases or open investigation the same as yours.

In the meantime, we can try a few steps to fix this unexpected behavior. Let's start with opening QuickBooks Online through a private browser. This is the best place to identify web issues. These are the shortcut keys:

• Google Chrome: press Ctrl + Shift + N 
• Mozilla Firefox: press Ctrl + Shift + P
• Internet Explorer: press Ctrl + Shift + P
• Safari: press Command + Shift + N

If you're able to add the second line item using the private browsing session, go back to your regular browser and clear its cache. We advise our customers to clear them out periodically. This refreshes our program.

Also, you can use a different browser like Google Chrome or Mozilla.

Please feel free to get back to this thread with more information or if you need more help.