Please post a screenshot of your timeline so that we can see exactly what you're talking about.
Also, what are your Timeline Display settings (from the Timeline Track Header)? There are several choices for the Thumbnail Display, None, All, First, First & Last, etc.. Could this setting be what you are having an issue with?
Also, with All chosen, it can take a bit for one's display to refresh, when one has gone past the first screen of Thumbnails. This will depend heavily on one's video card and driver, plus some CPU cycles. It does not affect playback, Export, or anything but the display on the Timeline.
First, work through all of the steps here http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with the DETAILS asked for in the questions at the end of that link