Are you pressing "display" on your DVD remote? If so, and 2.1 DTS is displayed on your TV, then you are playing a DVD with a 2.1 soundtrack. What DVD are you trying to play? Does 2.1 DTS encoded tracks exist? I thought that all DTS are 5.1 but I may be wrong.
Press "Surround" in your remote while playing the "DTS" or "Dolby Digital" tracks. If that doesn't not work, try pressing "A.Surr" or "DSP". If your press "stereo," your will only get 2.1 sound even on DTS and DD. I also have the Onkyo HTS 650 system.