While Old Major's teachings are spread around, the animals take no action until they are badly abused by Jones's neglect. Jones already has a history of drinking heavily, and one day he ignores the needs of the animals more than they can stand:
The men had milked the cows in the early morning and then had gone out rabbiting, without bothering to feed the animals. When Mr. Jones got back he immediately went to sleep... when evening came, the animals were still unfed... One of the cows broke in the door of the store-shed with her horn and all the animals began to help themselves from the bins.
(Orwell, Animal Farm, msxnet.org)
In a sense, this is the last straw for the animals; they are content to remain as animals serving the humans as long as their basic needs are taken care of and the humans don't seem to overly abuse them. When it becomes clear -- by Jones and his men using violence to push back against the hungry animals -- that they are seen as nothing more than, well, animals, the animals take action and drive the humans from the farm. It is this acceptance of token comfort that has kept the animals enslaved, and ironically, it is a similar acceptance of Napoleon's rule (to keep Jones from returning, as Squealer often repeats) that forces the animals into an even worse slavery.