I turned the missed blitzer up a little bit, we'll see if it works better tomorrow. It fired about 3 times for the week, which obviously is too low. My test last night had 21 blitzes missed over one week's worth of games, which averages to a little over 1 per game.
The missed blitzer probability is driven off of the experience of the players on the field. There is also a probability that a blocker will miss his block, which is driven by his pass blocking vs. the rusher's pass rushing attributes. All of the free rushers up to this last update were from the latter case.