Clearing the browsing history has solved this problem for me.
1) Open a new tab in your Chrome app and press the menu button on your Android device to access History.
2) Select 'Clear Browsing Data' at the bottom of your screen.
3) Only select 'Clear browsing history' and press Clear. You do not need to clear your cache, cookies, saved passwords, or autofill data.
4) Exit out of Chrome and also kill Chrome's application process. On most Android devices, you can do this by long pressing the home button to bring up a list of active apps or task manager.
5) All bookmark thumbnails on your widget should now be clear. If not, then enter and exit a bookmark folder on your widget. If you do not have folders, then open and close Chrome. Basically you need to refresh the screen. If your old thumbnails have not cleared, repeat step 4.
6) Now rebuild your bookmark thumbnails by visting each bookmark from INSIDE Chrome. Visiting bookmarks directly from the widget will NOT work. Open a new tab and press the menu button on your Android device to access Bookmarks. Visit a bookmark and allow the page to load completely. Press the menu Button again to access Bookmarks and repeat as needed.
About 22 to 24 thumbnails will be created assuming after you cleared the History, you only visited your bookmarked pages. After a certain thumbnail cache limit is reached within Chrome, it will stop generating thumbnails for your widget.
Through many tests I've come to determine that Chrome is creating thumbnails of pages in your History regardless of the page being a bookmark. So if you visit 20 non-bookmarked websites, and then 10 bookmarks, only the first few bookmarks' thumbnails will be created on the widget. However if you revisit one of the non-bookmarked website and bookmark it, then you will see it on the widget with a thumbnail.
I hope this works for other people and that my explanation was helpful.