There's a setting in photoshop to save a preview with the PSD file. I believe it's in photoshop under preferences > "File Handling". PSDs that were saved without a preview will not display correctly in most programs that reference it.
Since you're storing externally, you made sure the path is correct, right? IE you can export the file and open it fine in photoshop? My only other thought is that there's an incorrect reference caused by someting. Also, are you removing any special characters out of the PSD filename? Not sure if that has an effect or not.
I'd investigate to see if there's any pattern in those 1% of records, IE filename characters, user that created the file, etc.. That might help point to your quirk.
I’ll check that preview setting and see if that might be the issue. The path is fine. I can click on the container and export the image fine and when it opens, it displays the image. Just the thumbnail is showing all white. I’ll let you know if that was the issue.
The setting you probably want to turn on in Photoshop is "maximze backwards compatibility." This saves a flattened version of the all the layers in one layer and enables previews for asset management programs.
Turning the pref on also increases the file size.
Also important if you're opening files created in a version newer than Photoshop 3 (not CS3, Photoshop 3).