I don't know if this is too basic, but we tried to make butter from scratch one time and everything we saw said that if there was even the tiniest bit of liquid left when you're done churning, it would cause the butter to go rancid basically overnight, and sure enough, that's what happened the first time! The next time we made sure there wasn't any liquid left and it lasted much longer. I don't even know what the liquid part is called, but it would get stuck in tiny little air pockets while you were churning and it was very difficult to ensure that it was all gone by the end.