What version of AE/ Bridge, what presets and what computer? Only the presets that come with AE can be previewed in Bridge, as it requires some special trick on embedding a preview in the preset. Also note, that you may only be able to preview presets that match the version of Bridge, i.e. you will not have much luck trying to preview CS4 stuff in older versions of Bridge (though, for some presets it still may work, but not for all of them). One thing to definitely check is whether your Quicktime is configured correctly. Bridge uses QT functions to preview animated clips, so if that is not configured correctly, you may not see anything. In particular check the hardware acceleration functions in the QT panel as well as support for legacy CoDecs...
Is preview working for other types of files in Bridge? ie - images?
Are any Effects and Animation presets displaying? Is it certain presets that don't display?
The solution was missing quicktime. After I installed it, the previews worked perfectly again.