I ran across a problem for which I found no documented solution, but inadvertently found the solution myself. So I wanted to document this here for others who may encounter the same problem.
I was using a CAML query within SPServices to retrieve list items, then referencing "ows_{fieldname}" as usual to retrieve the field value for each list item. The field value was reported as "undefined" for all items. I quintuple-checked that I was specifying the proper name of the field and that my query was properly constructed.
I eventually discovered that the fields I was trying to reference were not displayed in the default view of the list. As soon as I changed the list properties to include the fields in the default view, the proper field values were returned within my javascript.
I don't know how much of Sharepoint's underpinnings work, but I was very surprised at my finding because it implies that the SPServices GetListItems method gets its data from the list's default view, not the actual underlying list. I realize I could be wrong in this interpretation, but I imagine others could run into this same problem.
The default, although unreliable, way GetListItems works is that it's suppose to returns all fields displayed on the default list view if you don't specify ViewFields on input. This is unreliable. Although you got it working - now - someone could come along later and change the default view thus breaking your sweet customization. Bummer. :) I suggest always listing the ViewFields you are interested in on the GetListItems method. This ensures that your will get them if they are set. That's right. You could still get rows with an undefined field. This happens mostly on fields of type Lookup that are not set on a row.
Hope this helps you understand what is going on. Over the years I have created my own wrappers around SPServices to ensure I get back a reference to all fields requested, even if they are not in the response by Sharepoint.