I have a few custom modules. Some are showing in Portals and other are not in the list to be able to be shared. All the modules have the correct sharing turned on. 

I noticed with the Quotes module, that when I moved the Owner field out, it didn't show in Portals. However, I dragged all the fields into the modules and the ones that didn't show still didn't show.

Is there a limit on how many custom modules can be shared in Portals? Or is there a particular field that must be used to make it work?