Sounds like you have bad luck honestly. I've run plenty of screens against zone. You just have to pick up on what the D's tendencies are. Do they run man more times than not, in certain situations, etc. etc.
I began using these in a new franchise I started. The other one I use a different set, but decided to just run with these to change things up. Lost the first game 28-17 to the Dolphins with the Redskins, but have since won 5 straight. My main issue right now is it seems a little too easy to run the ball. Overall on the year Morris is only averaging 4.4 yards per carry, but there are almost zero negative rushing plays. Last game I had 20 carries for 78 yards when the long was around 7 or 8 yards.
I'm just curious as to how much you all believe I should drop user run blocking? I would like it to be that even with a great line those 4 yard runs take some effort, but in a realistic manner still. Right now it just seems if I call anything from a toss to a stretch, dive, etc. that Morris is 3 yards in before he is touched and if he breaks a tackle he gets 3 more or he doesn't break it and falls forward for another yard or two.