Do the crops you try to assign them to indicate that they need a worker? Have you tried planting new crops and assigning the workers to those?
You can go over 18 food. I have some settlements with 22 settlers and generating up to 30 food.
Each assigned farmer can harvest 6 points of food; most foods only provide 0.5 food, except for Mutfruit, so typically a farmer can handle 12 plants, or 6 Mutfruit.
If there is an limit to the total food a settlement can produce, it Is certainly higher than the 30 suggested above, several of my settlements have 40+.
To maximize food production, ensure your crops are planted in groups of 6 total food produced(6 mutruit or 12 of any other plant) and that each group has an assigned farmer; use clothing to distinguish them from other workers for ease of identification.
I think it is an issue with that farm. There are a whole range of plants there that do not highlight in workshop mode. I put in extra plants, and don't forget the others at the other side of the settlement.
AF is one of my main food producers, not having any problems with crops there.
nm that's what i get for not refreshing before posting