I saw a youtube video where we can increase the rpm of mini dc motors that operate on 9v but changing the windings with coil having more thickness . I did the same as shown in the video but whenever I connected the power supply the motor no longer spins.. it vibrates for like maybe half a second and stops..the rotor rotates a bit then pauses.. proceeds to rotate a little again then pauses.. and the same is repeated..but mostly it stops all the time and I have to spin it with my hand again to get it to move a little. Why could this be happening?

 

I saw a youtube video where we can increase the rpm of mini dc motors that operate on 9v but changing the windings with coil having more thickness . I did the same as shown in the video but whenever I connected the power supply the motor no longer spins.. it vibrates for like maybe half a second and stops..the rotor rotates a bit then pauses.. proceeds to rotate a little again then pauses.. and the same is repeated..but mostly it stops all the time and I have to spin it with my hand again to get it to move a little. Why could this be happening?

Because you made a mistake in selecting the materials or doing the modifications. We were not there and were not able to observe what you did and/or what kind of materials you used. Some close-up photos and a video of what the motor is doing might be helpful.

ETA: Not everything on YouTube comes with a money back guarantee.

 

Using thicker wire results in fewer turns and lower winding resistance. Lower resistance draws more current. It is quite possible that your 9V supply can't provide enough current to run the motor properly.