Hi there, Cathrynhardy.
You should be able to edit the invoice. I've checked our file and didn't see any cases similar to yours. This can be a web issue.
First, let's open QuickBooks Online using a private browser. This is the best place to isolate web issues. If you're able to edit the invoice from there, go back to your main browser, then clear the cache. It's always a good idea to clear out the browser's history. This will prevent the browser to behave differently.
In addition, using a different browser fixes issues like this (Google Chrome, Internet Explorer, or Mozilla Firefox).
Let us know if you have other concerns. We're just right here.