The south wall is opaque, I.e. no light transmission. The top is angled to the south, but is angled far less than is required to receive the sun directly. So during the winter, the sun strikes your glass at and angle and most just bounces off. Also, only about 50-60% of the glass surface area actually receives the equivalent of direct sunlight, because the angle is wrong for the winter time sun position. During the winter, the south wall, which has no glass, actually receives more direct sunlight than the glass surface on top. What I'm saying is that with most cold frames, the front, the sides, and the top are all made of transparent materials so as to maximize sunlight and heat. If trying to maximize sunlight during the cold months, when the sun is low in the sky, you might consider replacing the front opaque panel, with a transparent material such as glass or twin wall poly carbonate. But if, as you say, that you only use the cold frame in mid to late spring, the benefit of doing such may be marginal, as by then, the sun angle is well above its winter time lows, and during April and May is striking the top transparent lid more and more directly as spring proceeds.
If by measurement, or by pant growth result, it is evident that the cold frame is receiving enough light, then obviously there is no reason to make any kind of change.