I just purchased and installed the Google Analytics +

I just purchased and installed the Google Analytics + plugin. The main statistics page works fine, but when I open up the post editor page for older posts and click to load the statistics for that post I get a graph with no information on it. I've tried this on two different sites and get the same result, a blank graph, as if that post has had no traffic... which is not correct. Am I missing something or is this a known issue?