Especially the GSO fabricator but the others suffer from this issue too, when the fab crafts a part where it is ejected to isn't terribly reliable to make picking up the output easy. If you try and use scrap magnets the block often gets caught by the SCU too early. If you just use SCUs then often the SCU will try and pull the object through the fabricator. If you build a carefully designed shroud round your fabricator to try and push the created blocks in a specific direction then the fab pops the block out through the shroud. In short the created block should always appear at the same spot and be pushed in a specific direction so you can position whichever block pickup system you want to use precisely.