Are you getting any error messages when trying to edit and add items in QuickBooks Online? Let me help you do some troubleshooting steps to isolate the situation encountered. Have you tried using a new incognito (in private) window? A private browser does not save any activities, which is why it's a good start to use it for any unexpected behavior in the program.
Here's how:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Safari: press Command + Shift + N
If you're able to edit and add new items with out an error, then go back to your default browser and clear your browser's cache.