Thank you for the suggestions - I had not used calibre in over a year and updated it from a 0.8.x version when I went to install new books on my Kindle. Your suggestion below fixed the issue - when I uninstalled calibre and then reinstalled it Kindle Collections worked perfectly. I did not have to uninstall/reinstall the plugins.
Thanks for your help!
Originally Posted by edwdecarlo
Another thing to consider, which version of calibre you were on prior to upgrading to 0.9.28. If it was a big enough jump, or you went from the 32-bit version to 64-bit version, you may need to uninstall calibre and re-install 0.9.28 again.