I agree with RyanH. Joining a network your revenue will be cut less. Freedom for example will take a 40%, YouTube will automatically takes 40% no matter what. So in the end you will be left with 20% of what you make.
Furthermore regarding why you don't earn that much, you might wanna check and see none of the videos are breaking copyright. For example If use the music by well developed artist even if its only for 10-20 seconds, the monetizing earnings for the entire video will go to the developer of the song you used.