Hello Lesley,
Believe it or not the most common fault is people putting the needle in the wrong way round, I would definitely check that first, it the needle is not picking up the thread then that is the cause, if it is picking up the thread on every stitch then it is either a threading problem, tension problem or bobbin problem.
Ken
drivewithken@blueyonder.co.uk
Ken Dourof, June 2007