Had the same problem, changed the needle, rethreaded - no change - finally took it to the shop and the repairman
indicated I must have broke a needle and affected the timing - he fixed it and now it is sewing like a dream again.
I didn't change the zipper foot when I was finished sewing in the zipper, turned off the sewing machine, next morning
turned on the machine and whoops the needle came down onto the zipper foot, and I hit the foot - learned a good lesson. Now I will change the needle after finishing the need for a different foot.