Hi Derek,
This is probably due to the fact that your Preview-marker has not been set to the correct frame.
If you look inside the Scene Director you'll see on the track with all the framenumbers on it, that there's a marker with a "P" in it. That the Preview-marker. You can position this anywhere you like in the animation and it will show you that image in the Scene thumbnail window.
To set the Preview-marker, position your mouse inside the timeline-part of the Scene Director where you can see the framenumbers and right-click on the position where you would like to insert it. You'll see a popup stating "Set Preview frame at Position Marker".
When you did that, the Thumbnail should be updated automatically.
Good luck,
Kenneth
#XPression