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?