If your smoothing pad has begun to turn white (especially along the edges of the disk), then you are beginning to trap ground glass into the underlying mesh of the felt substrate. Over time, this ground glass can build up to a point where it is covering the diamonds in the resin of the pad.
You need to use a Smoothing pad brush to clean out the ground glass from the pad.
Simply turn on your machine and put as much water as you can flowing on the smoothing pad.
Then take the smoothing pad brush and run it from the center of the pad to the edge of the pad. The bristles of the brush will work out most of the ground glass from the underlying substrate and get your pad working again.
Be sure to flush the pad extremely well with water after using the brush to make sure and remove any and all contaminant from the pad.