You have a great looking machine, and welcome to the Board. To clarify, the bobbin thread is being picked up and pulled up through the needle hole, and when sewing no stitches are forming? If that's the case it sounds to me as though the pointed nose of the shuttle isn't grabbing the loop formed by the needle thread on the upstroke. If you changed the needle and it is not oriented correctly it won't form a loop, you can try rotating the needle 180deg (if you have the flat of the needle shank to the left turn it to the right, if to the right turn it to the left) and see if that helps. If your upper (needle) tension is too tight this could affect the loop formation, you've already adjusted the upper tension, make sure it's looser as you're working on this.