But , if the iron core is the case, I wonder how many 3d printed motors work as shown in youtube!!
You're right, there are working 3D printed motors on YouTube. These motors are made with no typical iron cores in the coils or back iron next to the magnets, but the trade off is much less power for the amount of expensive copper and magnetic material used. Also, these motors are much more prone to overheating with even small overloads. So, these motors generally come under the classification of novelties and fun, but not very useful toys. Is this what you were intending to make? Or, did this just look like a great application for plastic based 3D printing? In design work we call this a solution looking for a problem to solve.
It's funny, I am knee deep in researching motors for an application right now. I saw this post and went right into researching 3D printed motors. I was taken in by one slick YouTube video on a 3D printed 600W motor. Until I realized a comparably sized motor, with the about the same amount of copper and magnets but with proper iron in the coils and back iron, would put out at least 3-5kW of power! In my opinion, this is a step backwards in motor design. But, it almost had me fooled!
Lorcx, this is of course unrelated to your questions. As brian relayed, I think you need to recheck your winding layout.