the problem is not with he bucket Nor with the milk, it is a problem with the bots thinking they can complete a job some other bot just finished, but instead of just trying again (as there is no way to detect you got milk in the bucket or not), it skips the command.
I found 2 solutions you can do yourself,:
1) only have 1 bot per cow ranch to milk (which will max be enough for 7-9 if you have them next to the milk storage)
2) you can make the bots try to milk multiple cows.
the second option is rather inefficient, as adding more bots means more tries to milk, which only speeds up the process of movement.
how I did it:
fenced up a 6x5 area (on the inside) and put a milk storage and a fertilizer storage on one of the sides filling half the space.
get 2 general storage, fill them with straw and have 1(6 cows) 2(7-12 cows) bot export the straw to the ground, next to the milk storage input.
1 bot milking. (tbh. I think they produce to much milk)
1 bot gathering fertilizer.
2 bots rewinding and walking out of the area to not stand in the way.