As you press your fingers onto the cone, you are exerting a force. As you squeeze, you also exert friction forces. If the frictional force is greater than the weight of the cone, it will lift out of its hole. As the angle of the cone gets larger, it becomes harder to grip the cone because your fingers slip upwards and you apply less force. In addition, as the angle increases, less frictional force is directed upwards making it difficult to lift the cone.