What model Raymarine John?
The earlier ones were actually JRC's with a different badge. The trackball is either rubberised and runs on two stainless rollers or is smooth and runs on two rollers that have a rubber tube sheath. These have a small metal disc with holes in them on the end that runs in a infrared encoder. There are a few things that can happen - the ball can lose it's grip, the rollers can become fouled with dust and not grip the ball, the rubber sheathing on the rollers can become mushy and flat spot (usually caused by contaminants), salt build up can seize the rollers or the encoders can fail - regardless the unit will need to be stripped which will involve removal of the lid, then the front face and finally the trackball PCB to expose the mechanism and find out whether the issue is mechanical or electronic. A basic strip and clean is often all that is required.