Occasionally you will see a Revit thumbnail that appears as the Revit logo and not an thumbnail of the family (image). You can check to make sure this is not an AVAIL issue by looking at the same file in Windows File Explorer thumbnail view. If you see the same Revit logo image you know its a Revit file issue. We are not completely sure why this happens but it appears to be a Revit file issue and not an issue with the manner in which AVAIL generates the thumbnail. Remember that we use the underlying Windows OS to generate the thumbnails (custom thumbnails are a different matter!).
So what's the fix? You can open the Revit file and generate a new thumbnail using a different view and/or in some cases, updating the Revit version will help (if that is works with your workflow).