Microsoft is working to fix a bug affecting all add-ins that cause them to stop working if a user clicks off of Excel.
More information on the Microsoft bug is available here.
The issue is reproducible on the following versions of Excel for Mac
Meanwhile the workaround is to ensure that Excel remains the app in focus, ie you should not click off Excel to any other app while operations are in process. If you do (or must) click off Excel the app will suspend operations and resume them when you return.