In Chapter Five, after expelling Snowball from the farm, Napoleon makes the decision to put an end to the Sunday-morning meetings. Instead, he tells the animals that all decisions will be made by a special committee of pigs, presided over by himself.
When the animals are told of this decision, they are very upset. Some animals want to protest but cannot find the right words, as we see in the text:
Several of them would have protested if they could have found the right arguments.
In contrast, some of the piglets do protest, but are quickly silenced by Napoleon's dogs:
But suddenly the dogs sitting round Napoleon let out deep, menacing growls, and the pigs fell silent and sat down again.
Finally, to ensure that there are no further protests, the sheep begin bleating, "Four legs good, two legs bad."
What we find, then, is that the animals do not protest because they are either lacking in the necessary intelligence to form an argument, or they are so bombarded with Napoleon's propaganda techniques, like the threat of violence, that they are forced to back down.