NOTE: the Valar didn't sink Numenor. Eru did.
The Valar (and Eru) didn't act until Pharazon completely decided that he wants to wage war against Valinor. There were warnings for him while he still was in Numenor. He didn't heed. He was given a last chance, and he almost took it - just before he stepped onto Aman - but pride was stronger. It's his pride that killed him.
I have to agree that the Valar are fairly inconsistent, with other issues as well. For example, they give Tuor immortality when they can't give it to Beren.
I guess that they think they are too uninvolved, so they stick their noses in, decide that they are messing around too much, go back into seclusion...