The add invoice button has disappeared from my iPhone app. Has there been an update to make the app even less user friendly?

I_hate_slowbooks, thanks for posting on the Community page, can you try refreshing the data in the app and see if that resolves it. 

 refreshing the date in the app, ios 

1. Go to menu (three lines going down in a line) and tap Help & Feedback 

2. tap Refresh Data. Note: A message appears "Caution. If there was a problem getting your latest Quickbooks \Online data, you can refresh it now. It may Take a few mintues" 

3. Tap Refresh to confirm

If still have and issue please try fully uninstalling and reinstalling the app again see if that resolves it for you.