You’ll need to change the improper way your theme is rendering readmore. Basically, instead of doing a proper hook to readmore, your theme is hitting it with a hammer. It “works” but it’s not done in the proper way, so plug-ins may have problems.
To fix this, you’ll need to modify your theme (the proper way would be to create a child theme from your theme). Then, correct the theme so it does a proper WordPress readmore. You’ll need to be a programmer or know how to do this of course.
To test, simply use a default WordPress theme, and then the plugin will work, which shows that your theme is designed using improper methods.
Tim