Bernettes are not highly regarded for quality. I worked on a cheapie WalMart Brother that had a very similar nylon bar mechanism. It similarly wouldn't stay on track. I do think there's something amiss with it. Like you said, a piece missing, broken or ? The Brother I worked on eventually got chucked. There ARE machines that simply can't be repaired and are disposable. This is why they're sold and made so cheaply. This is one of the main problems with these kinds of machines. The other is bent needle bars. Typically, in well made machines the needle will break if there's a major snag or problem. With these machines, the needle bar metal is weaker than the metal in the needle and the bar will actually bend rather than the needle breaking!