If you open it up you can see where the brass gets build up from transfer at the contact point. This is from the heat/arch of electricity and causes a small amount of brass to stick to one side. The part that has stuck has been annealed so it is softer and causes the sticking, either file the transferred part back off or replace the piece altogether. I file mine off several times, before it becomes necessary because of loss of material, to replace the brass. A spot of electrical grease slows the arching down as well.