Synthesizers are not broken, they always work. It's just that they cannot give something to a planet that already receives that material. For instance, if a Bot--> food world receives one bot, one organic, and exports three food, synthesizers won't work, because the resources it desires are already being provided, but it wants more.
Example of it working is a hive world that receives nothing. Plug in a synthesizer, and it receives food. If they worked the other way, they would be ridiculously overpowered, was a really balanced build.