I struggled with this for a while today. I was using SPServices’ GetListItems to pull data from SharePoint. I was using an external content type, and for the life of me could not figure out why the query was only pulling in some of my columns. The others just weren’t there. So I searched the web for a while, coming up with nothing that seemed to pertain to me. So I decided to look at my data. Turns out those columns didn’t have any data! So, in short, look at your data! I was using SPXmlToJson, as well as just the standard $.each() function to try to pull those columns; but with no data, nothing was coming back.
Bottom line: look at your data first!