I just had the same issue as described on this thread. I created a Hosted Feature Layer in ArcGIS Online. Added some fields to the layer but when I accessed the map on Collector the fields were nowhere to be seen. I double checked to make sure the new fields were editable. That was good. Then I figured it out. You have to open the map in ArcGIS Online and Configure the Popups! Once you check mark the boxes to Display the new fields in the Configure Attributes section of the Configure Popups menu, then the fields are visible and editable in Collector. Hope this helps!
