Adobe Bridge CS4 Previewing problem

It's been over a year since our department started using a central image library and, with only 2 or 3  minor issues, the print and web teams have experienced few problems accessing the photos using Adobe Bridge CS4. All machines have undergone both initial and repeated reindexing have produced no unexpected anomalies. Recently, however, several machines have begun to experience problems in loading previews of several recently uploaded folders.

Remember, our image library consists of a series of folders (by date), each with a varying number of images inside (but rarely over 2k or more images). Typically, once a folder of photos is cached (or "indexed) in Bridge, the program creates a separate subfolder for the thumbnails and another subfolder for the previews, all within Bridge's cache folders. (Located in the user's Library on a Mac.)

Initial indexing/caching creates all the thumbnails and previews for that one folder. Subsequent "recaching/re-indexing" only adds any additional metadata that might have been modified since the initial caching -- new images are almost never added to folders within the library.

Now, to the issue.

Beginning several weeks ago -- the timing is, unfortunately, uncertain - I began experiencing a preview loading problem on both the benchmark machines. (Two Macs, one desktop one laptop that serve as the most accurate and up-to-date sources for accessing the library.) The problem was first noticed in the search results window: previews appeared pixelated in the preview window and often remained that way. In addition, when attempting to review (or use the slideshow) feature, the images would often remain pixelated for some seconds or not load at all.

At first I thought this was isolated and temporary. But I then learned that several others were experiencing the same problem. (Only Macs, though. The two Windows machines, not overly used but one kept up-to-date, seemed fine.)

Modifying the preview generating option in the Path Bar made a short-term difference on a couple of machines, but the option was already selected on a couple of other machines exhibiting the loading problem.

When I began investigating the caching behavior of the problem machines, it soon became clear that the problem was deeper. By examining the cache folders inside of the Library cache folder structure, I could see that previews were not being generated even when the folder(s) were reindexed. In fact, I even tried purging and rebuilding the cache for several folders and watched the preview cache folder. I could easily see that not all the previews were being built and indexed.

The question now is why is this happening? The corollary is why is this happening on just a few machines? All the machines, it should be noted, are of the same vintage, and of robust features, and sharing the same OS and software.