I totally agree withe Sebrita and thank her for sharing this important tip as I think a lot of sewing machine users fail to realise just how sensitive and finely tuned the sewing mechanism is. All of my Quilter groups that I have start off with the 1st lesson is how to clean, oil and look after their machines along with when and how to change needles, and which ones to use.
They are taught how to clean, oil and defuzz their machine, often but I also suggest at least once a year they take the bottom off their machine completely and give a good all over clean from below, you will be amazed how much lint, thread piece etc; get in their. Besides a good little torch, I really recommend a good quality tiny brush, similar to this
http://www.ebay.co.uk/itm/5PCS-7-6-L...item1c3734109d cheap but really well made, being for the medical industry and also a good pair of tweezers.
I do advise at the for the very last desperate measure to touch the bobbin tension screw but if needs be needs must, I normally advise them to have me there to supervise, just for a second pair of eyes.