NE555 power controller not working with big motors
Big motors seem to wreak havoc on circuits because they make so much EMI noise and specifically with your circuit, you're trying to shove some back EMF into your power supply during the off portion of your PWM. If you have a 12 volt lead acid battery around you could try using the battery just for the motor supply and then only connect the ground of the battery to the ground of your circuit, that should isolate the two enough to see if it works (the battery is big and dumb and can accept the inductive spike). If it works then your problem is that your power supply can't accept power back from the circuit (likely) so you need to protect your power supply.

It also may be worth asking, this is a DC brushed motor you're driving right? A brushless motor will not work... I'm not insulting your intelligence, just covering all my bases.