Are you entering the ISBN into the Mac Zotero magic wand field? How are you entering into the Zotero web version? If you have the items in the Zotero web version, can you not sync to get the records into your Mac Zotero? What are the ISBNs?

To developers and experts: Is there a difference in the wand look-up system between the local computer version and the online version? I've never used the latter.

@Markg69: Can you provide a Debug ID for an attempt that fails? That's all we need here.

@DWL-SDCA:
The ISBNs in question are 978-0-9899665-9-7 and 978-1-73450-679-2.
I entered the ISBN numbers via the magic wand, as I have done for many books without issue.
After entering the above numbers separately in my Mac app, I received the message “Failed lookup” both times. However, entering the numbers on the web was successful.
Entering via the web app did sync with my Mac app.

We can't help you without a Debug ID.

@dstillman: The reason I'm trying to email is because of this paragraph on the support page: "If submitting output fails, you can return to the Debug Output Logging menu and select View Output, go to File → “Save…”, choose Format: “Text Files”, and save the output to a file, which you can email to support@zotero.org with a link to your forum thread. It can be helpful to ZIP the file before emailing it.

978-1-73450-679-2 (which you provided the Debug ID for) wouldn't work — that just wasn't registered in any central databases that Zotero checks. There are barely even any results in Google for it.

978-0-9899665-9-7 works for me, so we'd have to see a Debug ID for that if that's not working for you.