What does the wheat symbol above a follower mean?

I have never seen this symbol in game, but usually symbols above follower's heads indicate they are unable to perform the task because something is preventing them from doing so. In this case, it is likely wheat, and here is why: Once you have built a farming settlement in Godus, wheat becomes a requirement for building Adobes. Do you have wheat in reserve? If not, you might need to use some of your gems to buy some. It is strange that the fields did not build, as that is probably the source of your problem and could be a bug.

Once you have farming settlements, it costs a certain amount of wheat (depending on the plot size) to build a new building. Wheat is also used to give followers a burst of stamina, and for some upgrades, and the number of wheat fields you have is counted to unlock certain cards. If you don't have enough wheat to start building, the builders will wait at the plot with a wheat symbol.

Farming settlements don't automatically start building wheat fields, but they do generate farmers. Rather than the blue builder symbol, the settlement should have some green farmer symbols. As with building new buildings, you can click the settlement to send farmers out to build nearby empty plots into fields. Building a new wheat field does not cost wheat.

If there are no empty plots close enough to the farming settlement, you won't be able to send farmers out by clicking the settlement. You can, however, still leash farmers from the settlement to send them to plots further away, which they will farm.

Each plot only needs one farmer regardless of the size, but larger farms give you more wheat every time you harvest it.