quoting Nessari, Illúbequía, Seattle, Cascadia:

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 :/

I've been starting from scratch, but have needed to do so several times- I can't make Sandbox open a lot of my simulations.

Yes, the going nova bit is obnoxious. Even just doubling the Sun's usual mass makes it go nova.

Also, no, even if you make Venus the mass of Earth, it still freezes over. Not so much, I don't think, that it couldn't warm up with some extra carbon dioxide- it's about -20 C on average. Adjusting its orbit inward does work- it's just below freezing at 0.6 AU. Also, it seems to be impossible to make a planet be at any distance you want- sometimes Sandbox likes to adjust it again for you, which is annoying.