Where you mentioned that this was older content, I'm betting that the missing thumbnail problem is because they are rsr format as hborre suggested. Right-click on any item or folder in your library and it will give you the option to open it in explorer (or Mac's equivalent) and you can check to see if it contains rsr files. If it does, you can use Rsr to png converter to convert them to pngs. If poser is open when you run it, you will need to refresh your library. There are two circling arrows in the very bottom right corner of the library that will refresh the whole thing.

As you install, if you find content with both rsr and png thumbnails, you can delete the rsr ones. Poser 11 can't use them. They only take up space.

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