Why cant certain midi tracks be used in multi timbral mode.  Even when changing the bank there is no sound as if it is on mute.  It is like a waist of a track.  On the mpc I would skip 1a record on 2a, but then i skip 3a with no sound and so on.  Can someone help me with a explanation.

If you do not have a PART assigned to a MIDI channel then there will be no sound on that channel. Since each of the 16 PARTS can be assigned to any MIDI channel, if a channel is not sounding in response to data - it is because you are not sending data on the channel to which it is assigned.

By default the PARTS equal the corresponding MIDI channel:
PART 1 = MIDI Ch 1
PART 2 = MIDI Ch 2
PART 3 = MIDI Ch 3
PART 4 = MIDI Ch 4
and so on though to
PART 16 = MIDI Ch 16

But, for example, if you assigned PARTS 1, 2 and 3 to MIDI channel 1, then obviously nothing will sound when send in MIDI channel 2 or MIDI Channel 3… Basic MIDI stuff…

You assign MIDI channels in the Motif Rack as follows:
Press [MULTI] to enter MULTI mode
Press [EDIT]
Press the [MUTE/SEL] button to turn the LED to OFF, this selects the SELECT function.
Now the four buttons with numbers listed above them will “select” PART parameters.

Use the PAGE [<] if necessary to select the “VOICE” screen
Cursor Down to “RcvCh” parameter (Receive Channel)
Set each as required.