Sounds like the same issue I had with my DX18 and AR20310T. I even sent the receiver to Horizon for service. They sent it back saying there was nothing wrong with it, and no further instructions.

Since ch 11 is X1 and 12 is X2, I expected ch 13 to be X3...BUT NO! Ch 13 is also X1, 14 is also X2, if you want X3 you need to plug into15; 16 is X4, and so on to 20 being X8. Go figure. This is not covered anywhere in the instructions. It took posting in the radio forum and a bunch of playing around to figure it out.

Spektrum needs to update their manual. Also they do an overall poor job in many respects on the instructions that come with their components.