The problem is that manufacturing improvements(mainly the so called "All Construction" improvements), such as the factory, have their <ImprovementType> or <SecondaryType> listed as "MaxManufacturing", but the Plasma Furnace-B has its <GiveBonusToNeighborType> (and <ImprovementType> for that matter) listed as just "Manufacturing." The Furnace is looking for improvements where the <ImprovementType> or <SecondaryType> is listed as "Manufacturing," but only special improvements like the Death Furnace, Coordination Temple, and Garrison actually has that under their improvement type. Unlike the case with Approval where "GoodsAndServices" and "Approval" are interchangeable as improvement types, "Manufacturing" and "MaxManufacturing" are not interchangeable, so you get no bonuses.
As far as the base game is concerned, I think it is an oversight on the developers' part since even the four or so improvements that actually uses "Manufacturing" as their type have "MaxManufacturing" as their secondary type. This means that the Furnace is literally only going to give out manufacturing bonuses to five very specific improvements that are all race/tech specific.
You can easily fix this yourself by opening the DLC7_ImprovementDefs XML file and changing "Manufacturing" to "MaxManufacturing" for the Plasma Furnace. I believe there are a few other bugged improvements in there as well, but I forgot since I already fixed mines.
Edited because I realized Manufacturing IS being used by a select few improvements as their improvement type.
