Removing .html from urls (404 error after permalink change)

I recently decided to change the url on my WordPress blog from year/month/name to /postname/. I set up a 301 redirect in the htaccess file and most of the links work okay.

However, I have noticed a few links are throwing up a 404 error and not redirecting. The links which aren’t working all end in .html even though most of the other links don’t have the .html. I have tried several codes suggested online to remove the .html and nothing has worked. If I manually remove the .html then reload the page it redirects fine but I need to find a way of removing the .html from the urls as visitors won’t know to remove this.

I would really appreciate any help with this problem.

Thanks.