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Some of the beacons are below the production window.
As you can see from the production window its producing exactly 300 plates / s. As there is sufficient fluid flow I would expect it to produce 503 as per the tooltip.
I see that the copper & iron plate casting recipes produce 2 plates at a time meaning that its impossible for those recipes to complete so quickly. Perphaps the solution is to make 2 holmium plates from 80 solution in 2 seconds.