How did you setup your splitters? If you set them up in serial that's your issue.
Say you have 4 constructors. So the first splitter splits into a constructor and into the next splitter, which splits into a constructor and then into the next splitter, which splits into a constructor and into the last constructor.
In this case you send 50% into the first constructor and 50% into the next splitter. Then that splitter sends 50% of that into the constructor and 50% of that into the next splitter.
Basically you end up feeding the line less and less as you keep dividing the amount going down the line by half.
What you would want to do instead is have a splitter you feed into, which feeds into 2 other splitters that feed 2 constructors each. That way it's perfectly balanced.