the needle hitting the throat plate shouldn't have to do with the foot. If it was a foot problem it would hit the foot, and usually you can't move a needle over far enough to hit a zigzag throat plate so either you have you single needle throat plate on instead of your zigzag or you are pulling the needle to cause it to bend enough to hit the throat plate. If you really love to do piping and the piping is not large you might want to invest in an actual piping foot as it will work better than using a zipper foot.