I have an ArcGIS Feature Service which contains 8 line/point layers that is being consumed by the Shortlist Map Builder AGOL map and in turn being displayed in a Shortlist Story Map.
Interestingly the feature service in the Story Map is viewable only in a desktop browser window but not a mobile browser window when opened. I've tried it on both an IOS IPAD and on safari and chrome as well as a Motorola phone in chrome browser.
I feel like it's a setting in the ArcGIS Server Feature Service but I've checked and don't think I'm missing anything. I've also noticed a message in the browser "not secure". Next work around would to upload it as a AGOL (hosted) Feature Service and add to the shortlist builder agol map, which is not preferable but a possibility.