Hey Laney,
I had this same problem and found out the fix. In my case I added a new Domain "DomainEditSource" and assigned it to a new field "EDIT_SOURCE". I noticed the drop downs weren't showing up and realized I needed to add "DomainEditSource" to each of the defined subtypes.
I Right Clicked my layer and went to Data Design > Subtypes. Then scroll to your new field and add the Domain in for each subtype, you don't need a default value. See my attachment below for what this looked like for me.