Cant get my KA-BAR (knife) sharp?

I just cant get the thing sharp for the life of me. It is the USMC Fighting knife with the plain 7" blade. I have an Arkansas medium/fine stone and a Spyderco Sharp Maker and I can only get it sharp enough to where it barely cuts paper. It doesn't make a clean cut. I have sharpened my six other knives successfully with both methods. I can get them sharp enough to easily shave your arm hair. But not the KA-BAR. It came with a pretty crummy edge and I sharpened it as soon as I took it out of the box which made it a lot better. I just cant get that razor sharp edge that I want. I've spent probably three hours total messing with the thing. I am considering grinding a new edge on it at this point. Anyone know what the deal is?