Sarah Hayes
I have been having the same issue. I think it could be a Firefox cross site security feature or maybe just a bug. Have you tried other browsers?
I have a website where we have had intermittent issues. The client uses JS code provided by Mailchimp to display archive newsletters. They can never seem to add it and be able to view the generated archive list. But when I try, after a couple of goes I usually can, but other times I can't. I've done all the usual things like reinstall Sourcerer, switch caching off, clear site cache and browser cache, changed editor, switched off editors, switch RSFirewall off, checked RSFirewall and server firewall to make sure that Mailchimp IPs aren't blocked etc. Plus the [[ ]] trick which had seemed to work last time.
If I add the code to a standard HTML page, I can see the generated archive list, so the code itself is working. If I view Joomla article page source, I can see the JS code is there, but no generated list.
They had problems again this week and this time, I couldn't get it to work either. Quite by accident, I realised the the localhost version of the site that I am currently working on, did have the generated list (in all browsers). Now although I am sure I've tried other browsers before, I can't be 100% sure, but right now, I can see the generated archive list on the live site in Chrome and Safari, but not Firefox, which is my default/go to browser.