Polishing with a ceramic lap is not well understood. The ceramic lap is made of corundum particles cast in a glassy matrix. When new there is only the rolling and chipping action of the diamond. If you wipe all the crud off the lap the diamond goes with it and you have to recharge the lap. With use, the glassy matrix between the corundum wears away.
Tiny pockets develop in which the crud collects.
In addition to the diamond particles rolling and chipping on the surface you have diamond particles rolling around in the pocketed crud and a few diamond particles lodged against the back edge of the pockets performing some scraping action. The consistency of the crud and the amount of diamond in it is always changing, making it nearly impossible to predict how your facet will polish. It becomes a constant battle trying to maintain the amount and consistency of the crud and the amount of diamond in it. These problems are further aggravated by the fact that occasionally a particle of corundum will break out of the glassy matrix and roll between the lap and your stone, producing a scratch. These agglomerates can become larger than the size of your polishing particles, and cause a scratch.