The cache tester tests the timestamps on the front page but there is a setting to turn off caching on the front page. Presumably this explains why it looks like the plugin 'doesn't work' for me.
Could you create a test for a page other than the front page, if you think there is an issue here. Or at least display a message that explains why 'it won't work' for people who have set it not to and they should just ignore the test.