Sometimes the tension on the top gets full of thread dust and the mechanism can get stuck. If rethreading the machine and oiling it doesn't help, take it apart (where possible) and give it a good cleaning. It's amazing how quickly lint can build up, especially at this time of year when we are all frantically trying to finish up presents. You know how it goes. If you didn't have a deadline your machine would be working perfectly. :lol:
Also, make sure the thread you are using in your machine does not say Hand Quilting Thread. Thread for hand quilting is different from thread made to go in the machine. It is wound differently and it is waxed, which is bad for the machine.