I recently sent a thank you note to one of my college professors via email and never got a reply from her. I have not heard from her in a week and being that she is retired I would have thought that as a courtesy, she could have replied to my email (i.e. thank you for your kind words). Is it normal for retired professors to not reply to these sorts of emails?
I wouldn't say that. Professors really appreciate the "thank you" messages but replying to them makes some of us a little uncomfortable at times. Sometimes we don't know what to say also.
One thing for sure: it makes us quite happy and sometimes a little embarrassed.
There are 3 main reasons why she may not have responded.
1. The email you sent was to an address she no longer has access to, or does not check, because she is now retired.
2. As Elkady says above, some professors don't know how to reply or potentially does not see a reason to reply.
3. She did ignore you.
1 and 2 are the most likely reasons here. It isn't uncommon for professors to not reply all the time. In your case, don't do anything. Sending a follow up email asking why she didn't reply will be considered rude.