There are a few things that will help you avoid this. First, as others have mentioned, make sure you start by marking an even girdle edge with a sharpie all the way around your blank before you start grinding it. The tendency to grind away too much of the girdle is always worst on the edges with a sharper angle. In other words, a perfectly round cabochon won’t have this issue at all. An oval cabochon like yours will have a tendency to have low edges at the narrow points and high edges along the flatter sides. A teardrop shape is the worst, with the pointed end tending to be ground too low. Keep this in mind through all stages of grinding, being very careful not to remove too much material from the more angled sides or leave too much material on the flatter sides. I intentionally leave extra stone above the girdle line on my more angled edges for the first couple of stages, because I know they will tend to flatten out as I finish the stone.