When the configuration looks correct with your Serializers and Deserializers, but the video from a camera still won't stay synchronized, double check that the camera is itself is configured to accept the FSYNC input. This can be checked by disabling the FSYNC signal within the SERDES configuration and observing the VSYNC continuing to be present from the camera to the serializer. Ensuring the camera is configured to accept FSYNC input should ensure the system being synchronized end to end.