Hi, this sounds like https://bugzilla.gnome.org/show_bug.cgi?id=603102
Many users have experienced a problem where rendering did not behave correctly when you loaded a PiTiVi project and tried to render directly, without seeking (moving the playhead) in the timeline first. That issue has been resolved in the development version.
Please test the following: try moving the playhead on the timeline after loading a project file. Rendering should then function normally. If this is the case, then your problem is due to issue #603102.