is the phone that you bought brand new or used/refurbished. If any kind of dust, dirt, or lint gets in down around the trackball and into the rolling magnetic parts, it slows down the function of the corresponding sensor (up, down, left, or right). "Breaking in" may be just packing any residue into places where it doesn't disrupt the function of a sensor. Lint build-up can really slow function down a lot. I've pulled lint out of a trackball that seemed like enough to fill up a grown man's belly button. It just keeps packing in and building up.