My guess is the requests to fetch the feature service definition are failing or timing out due to a backend issue. You could try opening developer tools in your browser, opening the network tab, and then clicking to add the item to the map. If everything works correctly, you should see successful requests. If it's not working these will return no data and not have a 200 response.
This is what it should look like:
If you consistently see this issue, I'd recommend opening a support ticket.