What makes the QBs go deep? I do think IG plays a part in it, but it is also about the coverage. If guys are covered too well, they get all dinky-dunky on you. There is really a balancing act here. I strongly suspect your high threshold is playing a major role here - guys are likely too tightly bunched and the CPU is reading that primary targets are too well covered.
I would run an experiment and drop threshold using the exact set you are testing - I personally find that 19 is an almost perfect separation but I also think that 40 works well too. Yes, it is likely to throw other things off, but I think the threshold could be your issue.