Have you tried looking at and exposing the hidden format attributes on the reader? If you route to Data Inspector do you see the values there? Are you using a workspace to do the translation or just a default translation?

There are 2 main issues with AIXM to GML vs other formats. GML as a destination, is very flexible in regards to complex schema and can handle almost any character such as '.' and data structure such as lists. Databases typically have problems with both and will convert '.' to '_' and just ignore lists. This is why typically you need to construct a workspace that renames fields from . notation to _ and indexes out the list elements you want to non-list destination attributes. If you still have trouble please attach your workspace and some sample source data here and I can take a look, or contact us at support and send an example there.