Most of the glass boards we’ve seen use a layer of glass that is about ¼” thick (5-6mm). A magnet at the surface has to reach across that distance to attract to the steel underneath. A modestly sized magnet that sticks well to a refrigerator or regular whiteboard won’t be strong enough to hold well to a glass whiteboard.
Bigger magnets are needed.
We certainly have bigger, stronger magnets that can hold. Unfortunately, picking the right magnet isn’t so simple. If you just choose really big magnets, they might attract to each other much more than the steel underneath. This can make an unpredictable, slamming-together mess.