hit mode, and then pad 7 (program). click F3 to go to the params menu, and check the "play" field in the upper right hand corner. if that is set to "mono", then every pad you press in that program will cut off any sounds that are currently playing from that program. change that to "poly", and you should be able to have the sounds from different pads overlap.
the "voice overlap" field is similar to the "play" field except rather than having all pads in the program be cut-off by eachother, this field only refers to whether or not the current pad will cut itself off.
you can read up on this in the manual. in the Program chapter these fields are explained under "Limiting the number of the voice in the Program", and "Setting the overlap of the sounds on the same pad (voice overlap)", respectively. as episode mentioned, you should also read up on mute group and mute target.