When students have problems getting a piece to fit and ask for help I will mark the glass with my sharpie and tell them to go take it off at the grinder. The picture below is a piece of mine that needed to be adjusted so I marked the overhanging area with my marker which you can see the arrow pointing to.
Alas, too many people go over the entire piece rather than just the area that has been marked with a Sharpie. Don't grind other areas of the glass-- just grind the area that has been marked otherwise you're piece will end up being too small. This may seem obvious but I watch this problem occur on a weekly basis.
Also, students are amazed to learn that the harder you press the glass against the grinder wheel the quicker the grinding process will go. You can file this information under 'Good To Know.'