Can't add photos thru API

I'm trying to use the MixBook API to add photos from my server. I can succesfully create a user and a project, but when calling photos.add with an array of 9 photos, I'm getting a response of 0 size array, which means that no photos were added. I also don't see the photos when I'm going to MixBook thru the landing page. How can I debug it?
Here's pieces of my code (in c#):
//create a new project
int projectid = proxy.CreateProject(api_key, user_id, "My Album3", themes[0].id);

//add photos to new project
//pictures is a List of my photos
photo_attrs[] photos_array = new photo_attrs[pictures.Count];
int n = 0;
foreach (xxxx pic in pictures)
{
//adding 9 pics to the photo_attrs
}
//when deguging, I can see 9 photos are sent, 0 ids return, no errors or exceptions
int[] ids = proxy.AddPhoto(api_key, user_id, photos_array,projectid);
//send the user to MB
string url = "http://www.mixbook.com/partners/landing/" + url_name + "/?pid=" + projectid + "&login_token=" + user_alias;