There are several parameters that can prevent fluvadec from properly loading :
First a capable GPU is required. We support Intel recent GPUs, NVIDIA and AMD.
Having proper drivers installed for the supported GPU. For Intel you will need the VA driver and libva, NVIDIA uses libvdpau and proprietary drivers, and AMD requires FGLRX/Catalyst or libvdpau and a recent opensource driver.
If for any reason the 2 points above where not satisfied the first time you tried using fluvadec it might have been blacklist in the GStreamer registry. If you satisfied the previous dependencies afterwards you need to delete the registry in ~/.gstreamer-1.0 and run get-inspect-1.0 again.