Are you on the same version of RC Firmware on the master and slave ? That can cause it .
Make sure all your other settings match as well, ( do a reset to the slave RC , I have a link on the very bottom go to post on the link #8 for directions ) like both Master and salve use Mode 2 for control . Also you do have the slave set as such correct ? Here is a video with some tips also
donnie ****also go to this link for some more ideas, may have to reset the Slave RC as well and clear any presets that may be preventing it from finding the master/slave go to post #8 from DJI jamie