1, the bottom line has an adjusting screw, the one that installs the thread shuttle, a very small screw - twist loose (can not be too loose), and then a bottom line of the regulator, that is, the one that has a spring, to see if there is no debris between the two spacers, clean up a bit.
2, and then look at the threading of the surface line is standard, there is no wear wrong. If you wear the wrong one, you can change it.
3, check whether the edge of the shuttle by the needle hair, fine sandpaper can be used to gently polish, it is good.