Hi Andrew,
Could you please confirm are you looking to fetch impression share stats aggregated by campaign level? If yes, you can
contentimpressionshare and
searchimpressionshare attribute of the
Campaign Performance report. If this does not answer your question, could you please share the report definition along with client customer id so that I can troubleshoot further? You could choose 'Reply privately to author' option while responding.
Thanks,
Milind, AdWords API Team.