A prevalent issue with people is - mirror glaze was too runny. Once you take the chocolate mixture off the heat and add the bloomed gelatin. Stir well until all the gelatin has dissolved. It is essential that you wait for the mirror glaze to cool slightly, cooling the mixture will thicken the glaze. You can place it over a bowl of ice water or just give it a few extra minutes naturally. The glaze needs to thicken and reach about 26 degrees C. This is similar to almost room temperature.