In some cases you are not allowed to subscribe to a report (have it send schedullled to you). The reason for this is logical but well hidden for us mere mortals. It took me a while to find it so here it is.
You cannot subscribe to a Report Builder report that uses a model as a data source if the model contains model item security settings. Only reports that use model item security are included in this restriction.
For more information: http://msdn.microsoft.com/en-us/library/ms156307.aspx