They look at the rooting profile under both low and high light conditions to ensure that the rockwool product is compatible with their irrigation strategy. Try increasing the column height of your rockwool substrate. If you use RB6 or RB10 growing blocks switch to a BAB-12. Most rockwools are fairly high in silicon oxides. After that the largesheets of material are cut and packaged into the rockwool products you see at you local store.
Slabs usually are either vertical or horizontal.
Typically roots tend to spread out more in a horizontal fiber which can be very helpful under stress conditions. This is not required with General Hydroponics rockwool products. This sounds daunting but actually is a good thing, because it allows you to control the environment in which the roots are growing and increase both yield and quality. First of all be sure that EC in your nutrient tank is the culprit. Compare EC values in your slabs or blocks to that of the solution.