To keep your sewing machines tension in tip top shape, you need to clean the bobbin area regularly.
My favorite way to do that is with these Micro-tip brushes by Quilt in a Day.
They are tiny and fit into the smallest spaces. You can bend them..and either wash and re-use them..or just toss them out when done.
They work so good that I also use them to clean my keyboard, and the vents in my car.