Keokuk, even properly heat treated can be a bear to work and I discovered that though two rocks may more or less look the same, they can be different grades. I was complaining about it just last week on the "other" website (link below). I have however worked out a deal with the rock and it seems to be cooperating now, but I use a 1.5" copper bopper. I have messed about some with a hammerstone to see if it worked better.
All I can tell you is to prepare the edge with light hammering, grind very well and lay the rock on your leg pointing up at least 45° and whack into it like you are trying to get to the middle of the rock. It is tough rock and some people say it is good beginner rock, but if that's true, then I am a beginner after a 30+ years of knapping. Like I also said, there are different grades and in the material I am using, the lighter stuff with very thin veins seems to be a lot tougher than the stuff with bacon like veins in it and generally more orange in color.
You can read about my complaint here:
http://paleoplanet69529.yuku.com/topic/56630/Keokuk-Question