Needle keeps falling out of Pfaff Creativ 2170

Question

I use this machine for embroidery and recently the needles have been falling out of their holder. I insert the needle the correct way and tighten the s***w but within a couple of minutes the needle comes out and I find the tightening s***w has become loose. The machine maks a clicking noise just before the needle comes loose.I presume something has worn on the machine. Is this a common problem? How can it be fixed? If it's a simple solution as I'd like to deal with it myself. It took Pfaff a month to deal with a relatively minor problem last time.