We have steam heat in our 1930s-era home and one of the bedrooms is much colder than the other two. Here is a picture of the radiator in the cold room:
The room is 12'x12'x8'. This radiator is 8"x25"x4.5" in size. It has four fins each with four tubes. The tubes look like they are about 0.75" in diameter.
In contrast, our other bedroom of the same size has a larger radiator and the room is well-heated. (If it matters, both rooms are on the north side of the house; the colder one northeast and the warmer one northwest.) The larger radiator is 14"x38"x7.5" in size.
From what I understand the ability of a radiator to heat is proportional to its surface area, but using volume as a ballpark to check the appropriate size, I get that that radiator in the cold room is about 900 cubic inches in volume while the one in the warmer room is almost 4000 cubic inches. That makes me think it's too small.
Otherwise, the radiator seems to function well. It hisses loudly once in a while, but it does warm up—just not enough to heat the room. If the size is not the issue, what else could be the trouble?