More than likely your RSS feed is invalid or your web server lacks PHP / cURL support.
I can get SOME stacks to work with my feed. Yuzool’s Feed stack works, but I don’t love the look of it. Feedslider works. It mystifies me.
Sign up for FeedBurner.com (google) - check your feed and let FeedBurner convert the feed - the use the http:// code from Feedburner.
I have a site with over 50 different feeds - and that all is converted by google.
remember http:// - not feed://
That’s what I have done. It’s a feedburner feed.