Hello Suzie,
If your machine is missing stitches on cotton fabric then it will need retiming which is common on all machines, if it is missing stitches on stretch fabric then it is the wrong style of needle, if you need to retime the machine I can attach instructions if you email me, type my address as you say it not as you see it.
Ken
drivewithken at blueyonder dot co dot uk, January 2010