We have a custom Outlook VSTO add-in. We publish to a location which installs based on version, and it has worked perfectly for years. For some reason, the new version we pushed today is not running properly. It installs fine, it appears to be running (i.e. shows in the list of Add-Ins, has delays ignored, and shows as active) but the icon that should show up in the ribbon for a meeting is not appearing. It does work on the developer machines. No errors are logged anywhere, including Application Event Viewer. The install package does seem considerably larger, but there are no massive upgrades, and the machines with the issues have .Net 4.8 as do the development machines.
but the icon that should show up in the ribbon for a meeting is not appearing
By default, if a VSTO add-in attempts to manipulate the Microsoft Office user interface (UI) and fails, no error message is displayed. However, you can configure Microsoft Office applications to display messages for errors that relate to the UI. You can use these messages to help determine why a custom ribbon does not appear, or why a ribbon appears but no controls appear. See How to: Show Add-in user interface errors for more information.
Also you need to make sure that images are available on the end user machine.