Scott,
Thank you for your concern, this is the parameter you asked for:
Analyzing this, I've done some tests and I think I've realised what is causing the problem. In first place, the services are only printed if they use a basemap of the same projection, so if I try to print some services (which are in my own projection) with a basemap of the default projection, they wouldn't appear in the printed map. Also, if you call a layer from a service individually, it won't be printed in the result, I'm still trying to find the reason for that.
You have any idea how can I solve this??