In the Collector app if you were logged in before you made the map you can pull to refresh in the apps map gallery to pull down the latest maps and changes to your online maps.
When you go to this feature layers detail page in ArcGIS Online make sure that editing is enabled on the service. You should see this down towards the details of the page. If it is not enabled, click edit and then you can pick the editing configuration for the feature layer. Then after you do that, resave the map now that the layer has the feature layer with editing and then pull to refresh in the app and it will show up.
If you are creating the map in one account and viewing it in the app with another account, make sure you have the map shared through a group where both accounts are members of.
Hope this helps.