Some while back, I bought a Bachmann class 37 to add to my fleet (I always do a "test run" on DC, especially if I am putting the loco back in it's box to use later and fit with DCC). I was surprised to find that, on taking the loco out of it's box again after some months and fitting a decoder to the 8-pin socket, it did not read back on the programming track, neither did it now work on DC! So I ran some tests and found that the motor was fine, the pick-ups were fine, but for some reason the power was not getting from the pick-ups to the motor via the circuit board (if the pick up wires and motor wires were connected directly together on DC the loco ran fine).
So, to convert to DCC, I whipped out the circuit board and hard-wired the decoder directly between the pick-ups and motor (taking care to follow the mantra "red and black to the track, orange and grey the other way") and the loco is perfect on DCC now.
The moral of this story is: all that is labelled DCC Ready is not always as it seems to be! However, with a bit of solder, patience and care, you can succeed!
I don't know why the circuit board would not transmit the power, but, as I had found an answer by taking it out all together, I didn't bother to investigate this further!
I'd just like to add that, in the three instances so far where I've had a problem fitting a decoder, the problem has been with the locomotives and not with DCC. Suffice to say, I'm really happy with DCC as it has advantages over DC - plus the loco problems I have solved have been as a direct result of going through the process of fitting DCC in the first place!