I don't think it's the steel in the Shun's....but this somewhat depends on what's happening. Some powdered steels (Reserve's use SG2) loose some of their initial sharpness rather quickly, but then retain what's left for a very long time. I don't have a ton of experience with SG2 to say that's the case here....but it might explain some of what's happening depending on how dull they're getting for you.
Now, if they're just flat getting butter knife dull, no, this is not the steels fault barring some fluke in the galaxy where all your Shun's have a bad heat treat which is not very likely.....in fact I'd say it's near impossible.
So, if they're getting very dull, as Mark said:
You're not cutting on glass/stone/concrete/marble/etc.....right?
You're not being a ninja.
Not cutting something that's dulling the edge.
Barring one of those, your most likely culprit is a burr/wire edge. SG2 is pretty fun with burrs.
In this video, at about the 4:25 mark, I pull the edge across a dry paper towel to feel for a burr. http://www.youtube.com/watch?v=tKYIAqG9 ... x7ahm_x1iw
It helps detect small burrs for me. The burr will grab the paper towel and tug at it as you pull the edge across the towel. Try this trick and if the edge is buttery smooth as it pulls across the towel, you need to remove some burr.
If you're completely burr free, you might very well have the edge sharpened too shallow.
However, I thought the EdgePro only went down to like 10 degrees (atleast the Apex model) which shouldn't be too shallow for SG2.
My thoughts any how. Good luck, and let us know if you have any further developments.