In our current playthrough, (world generated with 1.3.1.1 for sure, playing under 1.3.2.1, I think) , we are having a bear of a time with rare critters, specifically that they do not seem to generate. Ice golems, we've waited in rain(snow) and heavy rain for several game-days. Not a single one. Angry nimbus, again several days with rain and heavy rain, and nothing. Same with rainbow slimes, and I changed over a biome to be exactly what I had in a different game -- mostly snow blocks on the surface, with enough hallowed ice blocks beneath to get the biome. Nothing.

We are in hardmode, have killed all three mech bosses, have not taken on Plantera. I'm not sure what to do, other than just forge along. It just seems odd that we can't get Golems or any other hardmode rain-based enemies. We've reinstalled Terraria, and I'm loathe to start yet another new world, but this seems strange.

I've tried older worlds, like 1.3.0.8, and a few before, and they seem to generate fine. Is it something screwed up in 1.3.1.1? And any suggestions?

Do you have the lifeform analyzer or better?

Does the biome meet the spawning requirements?

Have you tried using a water candle/other enemy attraction?

Yes. Cell phone. Yes. I've even tried building a farming platform 50 blocks high, 250 blocks wide, and standing on top of that. Yes. That and Battle Potion simultaneously.

Got it!

It's a multi-player problem.

I loaded a game in single player, waited for the rain, they all spawned just fine. I exited, loaded that world as a server, joined, and they spawned fine. In fact, once it was "clear" by the weather radio (and visibly), I was still getting rainbow slimes and ice golems.

Turns out the rule for multiplayer servers seems to be if you start the server while it is raining, you get the golem, slime and nimbus all the time, whether it's raining in the world or not. If you start the server while it is not raining, you never get them, regardless of the weather.

Well done figuring it out and thanks for posting the solution here.