I created a web map which has an editable feature layer in it for our staff to use for offline data collection in the field. On multiple devices, we have been able to successfully download an offline map area in Collector Classic, but not the new Collector app. Since it seems this isn’t an issue with the map being offline capable, any other ideas why this might be happening? I did not receive any other error message other than a generic “download failed”.
Edit/Update: I have received a lot of helpful insight on this topic, and in my particular case, I believe the error was caused by having a copied layer in the web map. I was able to determine this by using the Developer Logging feature described below.
If this issue pertains to you, you can up-vote this idea: https://community.esri.com/ideas/16311-enableallow-maps-with-multiple-copies-of-a-layer-to-be-used-offline-in-collector
Here is a related Geonet blog post that I did not find in my initial research: https://community.esri.com/thread/227015-webmap-can-download-in-collector-classic-but-not-in-new-collector-for-arcgis
Hope this helps someone!