The illustrator(.ai) files can't display thumbnails
If your ai files can be viewed in Illustrator, the reason that it doesn't work well in Eagle may be caused by the file's version or compatibility.
How to fix?
The reason for this problem is related to the compatibility selected when ai files are stored. You can use an illustrator to open the ai files and save them with the "Create PDF Compatible File" option on. The compatibility problem is solved