A carbon skimmer will not function if its output pipe already contains 1 kg of polluted water

Your issue may be due to the the outputs of the pipes bridges, that directly surround the output of your carbon skimmer.

The pipe system don't like to have 2 outputs in a row. It's better to leave 1 section of pipe before having the second output.

Also, take a look at the content of the pipe at the output of this bridge, right to the skimmer. Maybe it contains 10 kg of water, If it's the case, it will block the flow of the liquid from your skimmers.

 

So I suggest to replace the bridge between the 2 skimmers by a pipe section, and put a bridge with the input at the middle of this new section.

Also, the vertical bridge on the right of the skimmers don't semme to have a real use.

Try something like that :

Edited March 12, 2019 by Gwido

Thank you, and especially thank you for spending time trying my thing out in sandbox.

It's easy to design a workaround but what I have just absolutely does not work, period, and this is not intuitive. The output pipe has 1 kg in it. It's not full. The skimmer on the right can't put 1 kg in a 10 kg capacity pipe if the pipe already has 1 kg in it.