These two placeable trigger are playing for the person you on the standart maps or where on the map the MilktruckTrigger are not installed!
The triggers for the wate rmilk trailer needed to fill from the cow pasture and then me to sell him with milk.
In the shop there then buy 2 Trigger:
MilkTruckFillTriggerPlaceable to fill the trailer he has a black / white mark.
MilkTruckStartTriggerPlaceable to sell the milk he has a Black / Blue marker.
Mutatis mutandis, one should place the FillTrigger in the cow pasture and the start trigger in the Molkerrei, but that is not a requirement.
You have to klinken with the mouse exactly to the spot to trigger re-sell.