I agree with all the other posts. Start with easy first, change needle size, make sure no lint in the bottom, make sure thread size is correct, make sure new needle is in all the way. Also make sure the needle is inserted the correct direction. Needles have a little groove in them below the eye for the thread to nestle in, which is important that it is in the correct direction for the loop to be made. That's why there is a flat side. If the needle is in the wrong direction, the loop won't be made. For example, the flat side is to the left on a 221 but on the far side for a modern machine. Last resort is to check the timing, that being the "when" of the needle picking up the loop. Good luck.