While you will be given full control over the forward lookup for your DNS within your netblock you rarely have any control over the RDNS to those same addresses since they effectively belong to your ISP and they usually keep control of the RDNS resolution rather than let their customers mess up a nice addressing system with random RDNS names.....

Hence, in your case they haven't set up the RDNS for your netblock and thus your can't RDNS your own netblock.