I don't know enough of the physics to say for sure, but I'm not exactly surprised. Venus is smaller than Earth, thus it's going to have a lot less geothermal heating going on. I don't know if its orbit is close enough to counteract that. You might try adjusting its distance inward to see if you can thaw the iceball.

Also have you found any better way of building a solar system than editing Sol or starting from absolute scratch? The pregenerated solar systems I try keep going nova for some reason :/