I agree with Chester the Bunny. A friend came into class with her just picked up machine after a service . Having trouble threading I went across to help. I threaded then asked if she wanted a bobbin put in. Delighted she said yes. I opened the bobbin casing to find it stuffed tight with fluff. It took a good ten minutes to remove and completely clean.
this should have been done as part of the service. It had cost £60. I got satisfaction in helping a friend who didn't know how to clean area. Her machine gets cleaned every bobbin change now.