There are a couple of components in steel that affect dross from plasma cutting....silicon and carbon content. On most grades of steel these components are allowed to vary in a rather wide range. If either or both of these are on the high side...the dross free zone gets very narrow or non existent.
That being said, the design of the torch you are using affects the speed range of the dross free zone as well. Torches with better designs will have a wider dross free zone on the same material that a different style torch produces a narrow or non-existent dross free zone. I know that Hypertherm process engineering teams now have strict specifications when developing new torch and consumable designes to ensure the DFZ on most materials is as wide as possible.
The PAC123 torch is of a design that is a couple of generations old. It is possible that you have some material that simply will not cut dross free with that torch....but may cut perfectly dross free with the replacement torch upgrade (the RT60 torch) that was designed for the Powermax600. The RT60 torch is the same torch that is used on the Powermax1000, it has a technology called "coaxial assist" that increases arc velocity as well as energy density. The very latest torch from Hypertherm is the one used on the Powermax45, it has another innovation called "conical flow" that takes the RT60 torch technology even further.
Some manufacturers stick with the same torch design for many years, as it saves R&D engineering costs. Hypertherm is committed to continuously improving the process, so it is typical for a new torch to be developed every time a new system is released. This upsets some owners, as overnight their systems can be considered old technology, but it is done in the name of progress.
Heres an old trick if you cannot get rid of dross: Spray the bottom of the plate you are having issues cutting with a mig welding anti spatter spray. Usually this will eliminate the dross if you have your speed and height dialed in fairly close.