Best check with the insurance company. Unless, by "transferred old NCD to new car" you meant you enlisted your SA to help do it and he hasn't done it yet then best check with the SA. Usually, if you had successfully transferred the NCD you would have received a refund on the old policy that was terminated, plus a separate refund on the premium paid for the new car amounting to the NCD percentage that was transferred over.

Most insurance companies take around 1-2 days to transfer the NCD and 30-45 days to process the refund if any.

How did you transfer the NCD - the procedure?
Your new car policy should state the NCD, and it will be the same NCD as your old car if you did the proper transfer.

I checked mine which was transferred in August this year, -under old car plate the NCD has become zero, whereas it reflects the actual NCD when I key in my existing car plate number.

I'd call the SA and current/previous insurance companies to find out what happened.