The following is the message I had written to a question regarding confirmation of a ticket for Delhi-Palanpur in Ahmedabad Rajdhani.
Rajdhanis suffer from a typical Distance Restriction and Restricted Distance Syndrome. This means you may not book your ticket for short distance and/or typically one of the terminal heads needs either to be the originating or destination station of the train (For most of the Rajdhanis-There are exceptions). Rajdhanis also do not have any sharing quota concept, where berths may be forwarded during the charting of the originating or previous remote station.
I...
more... would assume NDLS-PNU would have its specific quota, which again is very less. Now for your ticket to get confirmed, the confirmed passengers had to give away their bookings. Also you need to assume that PNR-ADI being a short leg, you would not expect much people to take Rajdhani to cover this distance/journey leg. Hence, even if an extra coach is attached, berths would not be allotted to NDLS-PNU leg unless and until system can find one passenger of PNU-ADI/Any Confirmed passenger for NDLS-PNU leg cancels his/her ticket. This is also the precise reason why in August Kranti, people suggests to book for NZM-BCT and not till BVI or ADH.