Yes. you missed one of the most important parts of a manual farm. they are manual, because you have to also plant them in the case of the rubber farm. it only harvests the rubber, without harming the trees. so it does not need a supply of saplings.
The wheat farm has to be manually tilled before any seeds will be planted. you basically have to tell the farm where to plant by tilling the fields yourself. this allows a manual farm to use it's area more efficiently than the managed one.
Last item that you should check. have you given the farm any fertilizer? it will not work without that either.
A last note. check witch order you put the tubes into the circuit board. from top to bottom it should be North, South, East, West
trying to plant wheat in the direction you put your rubberized tube into will not work.
If it still wont work, then check the GUI for an error message, (red tab to the right of the GUI) and post it here.