Have you tried to stream any kind of video with RTP? I mean, you should test the RTP stream with a sample video instead of trying to do all at once, that’s a good way to check that everything is working properly. Once you know that you are setting up the RTP stream correctly, then you can add the recording of the webcam to the RTP stream, but trying to accomplish everything at the same time can sometimes produce a lot of issues (at least that’s what happen to me).
You could try to do the example in RTP streaming with ffmpeg maybe you'll find there the difference between what's missing and what's not. I hope it helps, let us know.