Dashboard in an iframe not working on all domains

I’m having an issue that I can’t seem to figure out. I’ve got Piwik installed and three websites being tracked. On one of the sites, I was able to set up the dashboard so that the site owner could check their site stats. On site #2, I’ve put the exact same code, but the iframe just won’t show up. I’m thinking that it’s a domain issue, but I can’t find anywhere in the administration area to enable the iframe to work on a given domain. Am I missing some simple setting somewhere?

Does no one know how i can display a dashboard on another domain? This feature seems to work sporadically, and I can’t find any setting to allow or not allow this… please help!