A number of people report difficulties in sliding the glass along the surface grid. The glass catches on the grid squares and so does not move easily and smoothly when grinding.
Some suggest sanding the grid to remove any rough places. The difficulty with sanding the grid is that it will mean that you have to replace the grid before the grinder comes to the end of its life. Whether you will be able to replace the grid is a risk you have to take if you do this.
It is better to give all sides of your glass a quick arris before beginning to grind. Although technically, an arris is the edge of a piece, it has come to mean the modification of the edge in glass work.
An arris on the glass edges can be made by hand with a pass of a grinding stone on the top and bottom edges.
It can also be done by a light pass of the glass along the grinding head. This arris protects your fingers too, as it removes the sharp edges of the glass.
Make sure any points on the glass are slightly rounded, as they are most likely to get stuck in the grid. You can nip the point with your grozers, or give a slight rounding of the point when making the arris.
It is important that you do not press down on the piece of glass. Press horizontally toward the grinder bit instead. The top is plastic and so deforms pretty easily. With long pieces the bowing of the top means that the glass, which does not bow, will catch on the grid. So, to keep the surface grid flat, hold long pieces at the ends. This will remove any tendency to press down in the middle, as any downward pressure will be at the ends of the glass, allowing the grid to remain flat.