there could be many reasons why.
I would say:
1. check if your gearing is correct
2. check if your drive train is free spinning
3. use a deans connector instead of a "tamiya style" plug (a white plastic shell connector)
there could also be defective cells inside your battery pack. its been a while since nicd 2400 mah batteries were popular, it may be no surprise that these cells were sitting and that some cells may have become defective.
my personal guess would be gearing. Your car may not be properly geared for a 15T motor, because it was using a different motor previously.