If you have been using a sewing machine long, most know the result of not cleaning the lint. I would sew along and not pay any attention until it was too late. Either my stitches would get wonky or I would get birds nest. After a quick look, the diagnosis was simple... it needed cleaned.
Here's the Tip:
After you fill your bobbins clean your machine. I wind six at a time. After going through them, I have made enough lint my machine needs cleaning. Your machine is sure to be different than mine, so play around with how many you fill before it needs a good dusting. Always wind the bobbin before you clean or it will be futile!
Hope this helps!