It's usually because you have set Bridge to show embedded thumbnails instead of generate HQ thumbnails. Look at the lower toolbar on the right, there are two chequerboard icons. The left one goes brown if you are using embedded. The right one can be used to set your preference.
Failing that, you may have a problem with your thumbnail cache. Selecting all the images in the content panel with Ctrl-A, right-clicking, and selecting Purge Cache for Selection should force the thumbnails to be rebuilt. Sometimes this fixes it permanently, sometimes the problem will come back. You might need to do a bit of cache spring cleaning, by visiting Edit > Preferences > Cache and doing a Compact Cache and maybe increase the cache size.
Thank you for the tips. I tried all of your suggestions and unfortunately it is still not working :/
It might be worth resetting Bridge. Off the top of my head, I think this involves:
Quit Bridge completely (sometimes it runs in the background if it's set to do that).
Hold down Ctrl and click the Bridge program icon to start it again.
You should get a pop-up dialogue box with three check-boxes...
Tick all three boxes, and click OK.
Please bear in mind that this will take Bridge back to factory defaults, wipe the keyword index and thumbnail cache. You will be able to rebuild the index without loss of data and the cache rebuilds as you use the program—it's just a PITA.
If this doesn't fix the problem, there may be system-related factors affecting your problem. Check free space on your disk(s) and your graphics driver. Maybe do a disk integrity check too. Have you installed any other system software at about the same time, like new anti-virus, etc?