I understand that to be available for collection, a feature layer must be hosted (feature service) and editable.
I also observe that only maps are offered by collector that contain such a layer. The feature service as such is not offered if not via a map.
However, not all my maps that satisfy this condition are offered.
One thing I could imagine, and which would correspond to my observations is that data with an extent that does not include the current position is not offered.
What is the precise condition used to filter the maps?
The maps must contain and editable feature service. This is different then the map notes layer or imported files like gpx, csv and shp that can be added to a web map and edited in AGOL. The editing of those features are not supported in the apps today. Secondly the maps must be shared to you through a group or exist under "my content". The app does not pull on the web maps that exist in your organization.
Hope this helps