I found this article that seems to describe the reason why iCloud bookmark sync for Windows Chrome does not function as expected (if at all). He states that bookmarks are not sent directly to iCloud. Instead, they are synced with "transactions". But if iCloud receives a transaction for a bookmark that has already changed or been deleted, the sync fails.
If what he says is accurate, bookmark sync can stop or behave erratically if iCloud gets out of sync with Windows. This was the case for me.... I had synced a year or two ago and then uninstalled the Chrome extension. In the mean time, many bookmarks were edited / moved / deleted. So when I tried syncing again yesterday, the sync from Chrome to iCloud failed. So I tried the following:
With the Chrome extension uninstalled and bookmark sync disabled in iCloud for Windows, I deleted all bookmarks from my iPhone and iPad (still set to sync with iCloud). I then re-enabled bookmark sync in iCloud for Windows and installed the Chrome extension. Nothing happened (or I did not wait long enough) so I rebooted the Windows machine and after reboot, all my Chrome bookmarks appeared in the iPhone and iPad.