Google docs previews - some do; some don't!

Google docs previews - some do; some don't!

Sometimes google docs attached to a Nimbus Notes page show previews; sometimes they show nothing - ie just the boxed document link. There doesn't seem to be any reason that I can see behind this - it doesn't seem to matter if it's a long or a short document, for example. Am I doing something wrong, or is this a bug? (picture attached; two almost identical documents; one previews; the other doesn't)