In these two photos, you see a sewing machine that has a lot of lint build up in it. The lint in the first pic, is not in a place that is easily reachable by the user. In the second pic, you can see the build up in the feed dogs. This is very easy to access by lifting off the needle plate and then cleaning.
There was so much lint in the mechanism, that the plate was lifting up with each stitch that the machine made. Because of the impact on the underside of the needle plate, the feed dogs were pushed down and would not feed the fabric. As a result, they had to be adjusted to properly transport the fabric again.
If the customer had cleaned this area herself, the machine would still have had to be serviced to remove the rest of the lint and to be properly lubricated, but, her sewing would have been less frustrating to that point.
Innocent as it looks, lint can interfere with the proper functioning of a sewing machine and can create premature wear by wicking away oil where it is needed. Be kind to your machine and clean and lubricate where you can. Have it serviced regularly. You won't regret it. Cheers!