Apr 13, 2016, 19:5404/13/16
Greetings m'lord ,
You're correct, you should be able to build up to 6 Farms! Make sure nothing is in their way. If you build the Farm on a place where something is already built, it obviously will not work! Check if you have the sufficient amount of resources for constructing it too. It might be possible that your 6th Farm is already somewhere else hidden behind another building.
If nothing of these things work and the issue persists, please contact the Help Desk.
Good luck!