If you are using a DCS AIU unit, it is possible the ground wire going to the AIU switch position is not connected.. To expand, coming off the AIU you should have 3 wires for each switch. One going to the left side and one to the right side, then a ground to the middle terminal of each AIU switch position. (In my case I used one ground connection, then daisy chained all the "center" AIU connections for each switch, if that wire was to get disconnected then none of the switches would work.) An easy test would be to test by shorting the two outside rails together at a switch, (simulating non-derail) and if the switch works, it is most probably the AIU ground wire.
If you are using Remote Control from the hand held remote, and have the push buttons installed, and the derail. You should be able to test each option and find out which one is not functioning. Then investigate which one is the problem.