Would there be a reason a feature layer added to a web map wouldn't display in Collector app but does in Explorer App. I built a quick hosted feature layer to be editable and used via Collector. I then just added a feature layer to my web map for visual reference, addresses, that is already uploaded to AGOL coming from my ArcGIS server. I only have labels being used on that particular feature service and they never show in Collector. But when I add an existing map image service that also contains the same layer among other land base data it will display in Collector.
Using Collector v18.0.3 build 1033