The first difficulty the animals have in building the windmill is agreeing to build it. Snowball proposes the plan and designs it, but Napoleon is convinced that it would be too time-consuming and their energies should be spent on gathering food.
The animals formed themselves into two factions under the slogan, `Vote for Snowball and the three-day week' and `Vote for Napoleon and the full manger.' (ch 5)
Once Napoleon runs Snowball off, he is able to convince the animals that the windmill was his idea the entire time. He uses it to rally the animals and give them a goal. The problem is the actual building of the windmill.
But the problem the animals could not at first solve was how to break up the stone into pieces of suitable size. There seemed no way of doing this except with picks and crowbars, which no animal could use… (ch 6)
The animals solve the problem by dropping the stone down the hill, where it breaks into pieces. Then they can easily use the sand and cement they found to start building it. The animals work hard, and are very focused and happy. Unfortunately, the windmill is destroyed by humans or natural causes, and it never really gets built.