What kind of machine do you have? Did you ask the people where you purchased the foot?

I am going to try these things. It started doing this after I had to have the timing belt replaced, but if I have it looked at by a professional I am without the machine for at least a week! I'll try adjusting the pressure on the foot, and cleaning it. Thanks!

I have two walking feet. With one, the lever just rides on top of the screw that holds the needle.
On the other, the lever has a U shape on the end that just slides around the screw that holds the needle.
You do NOT screw the lever on with the needle screw.

can you post a picture?