I think I've maybe tapped on only one or two machines and gotten them unstuck. My latest was yesterday. It was a Singer 99. Bobbin area was covered in rust as was the bottom of the machine - big clue. The machine looked good on the upper part. I spent the better part of yesterday trying to free up that machine. First I cleaned up all the rust with Finish Line bike chain lubricant. It is for cleaning up rust as far as I'm concerned. Then I cleaned and oiled the machine multiple times with no movement. I could rock the hand wheel and see slight movement in the needle bar - that told me the problem was unlikely in that area. I spent a lot more time oil in with Tri-flow and rocking the wheel. I could not see anything in the upper part of the mache so blindly kept oiling. I took the hand wheel off, and used a good mechanics glove to help turn it. Just not budging. I suspected the bobbin movement from the start but a lot of times the area I suspect is really not the part that is stuck. I do not like to ever force a machine to move since something else could break, bend or nick. Gently rocking it while oiling, will more often than not, make the machine turn. But you never know so I oiled every place else I could thin might be a friction point. After I was sure everything was well oiled, I decided to try tapping. This time I needed to tap on the part that rocks the bobbin. I tapped a couple times. Rock and tap. Oil from upside down, side ways, front to back. I like to let the oil do the job. I tapped gently and all of a sudden it gave. I put the machine back together and added a hand crank and turned and turned the machine to work the oil into it at the same time cleaning dirty oil out and adding clean oil. Then I cleaned out excess oil. This time it was the little spot under the bobbin holder carrier stuck with rust judging from the ooze that come out of that area once it turned. the 201 I had that was stuck unstuck the instant I pulled the motor. It was a stuck motor. A lot of machines don't get oil in rods and their mechanisms just left of the hand wheel and they get dried up oil stuck or rusted in the places that should move in that area. I've seen rust or dried up oil causing it to freeze behind most of the parts behind the needle bar area. You may need to listen as you rock the machine and feel what might be loose. It may NOT be the needle bar at all that is stuck. I had a machine a very long time ago that I tore apart because I couldn't figure it out. Someone had taken the shuttle off and put it back on incorrectly some how causing the whole thing to freeze. I had to learn a lot about repairs the hard way. Since I am not a really strong person, I like to let the oil work and not get in a hurry to use force.