Yup just found this out as well. The Data Listing Widget is buggy. It worked fine before the March 2015 Update W
Work around I did is make a new group (ie OPEN DATA STUFF 2) in ARCGIS online make sure you check the Designate as avaiable for Us in Open Data. ADD any feature services you want to be in the Data Listing Widget. In Open Data Manager.
Go back to your OPEN DATA Manage Websites choose your respective open data website ---> Go to Manage Data Tab Then ADD the new group you made to the Selected Open Data Groups. Its a Temporary FIX but it works.
Did you know if this bug has been fixed