iTunes doesn't recognize updated iOS apps

I have my iTunes Library on an external disk, connected by Firewire 800 cable. Have used the same set-up forever. However, just since July iTunes can't find the latest version of apps that it just downloaded. Usually happens after quitting iTunes or restarting the laptop but will also occur a number of hours after updating those apps. I can download the same apps multiple times and always, within a day, iTunes will alert me that it needs to be updated. If instead I use Get info > Show in finder > Locate, then click on the correct app, iTunes removes if from the "Updates" list but it will return at some point within a day. Likewise, iTunes will sometimes (not always) download the same podcast episode 2, 3 or even 4 times.

Thanks for any thoughts.