Hi,
How are you assigning the "subscriberfullname" value to the macro resolver? Am I understand you correctly that you are sending the e-mail from a template using API?
Have you tried assigning the macro using this way?:
EmailMessage msg = new CMS.EmailEngine.EmailMessage();
EmailTemplateInfo eti = EmailTemplateProvider.GetEmailTemplate("MyTemplate", CurrentSite.SiteID);
MacroResolver mcr = new MacroResolver();
mcr.SetNamedSourceData("subscriberfullname", "Insert your name");
msg.EmailFormat = EmailFormatEnum.Both;
msg.From = eti.TemplateFrom;
msg.Recipients = "name@domain.com";
msg.Subject = eti.TemplateSubject;
EmailSender.SendEmailWithTemplateText(CurrentSite.SiteName, msg, eti, mcr, true);
In your template you can then use the macro and it will get resolved when you send the e-mail.
Hope it helps!
Kind regards,
Richard Sustek