Inconsistent Results With London Broil

My sous vide set-up is a 7 quart oval Rival CrockPot with an aquarium pump and two air stones circulating the water. I have two different controllers: a Johnson Controls A419 thermistor based thermostat and a JLD617 PT100 type PID controller. I get consistently tender, perfect results when cooking tri-tip, flap meat, beef rump roast, New York strip, sirloin, or boneless/skinless chicken. When I cook London Broil or round steak the results are inconsistent.

I fully realize that round is a cheap cut, that I can't expect the flavor of a New York steak from round, even sous vide. The shocker is that sometimes I get the unexpected: a 24 hour 131 degree F masterpiece to rival the best steak I have ever eaten anywhere! They are seemingly redder than the 131 F degree tri-tip I get consistent results with. I get that about one time in two or three attempts. The rest of the time, the London Broil will be tougher, chalky dry, showing every sign of having been overcooked. The first time I tried 12 hours it came out superb. Later, 12 hours turned out to be typically tougher on average. Looking for consistency I have tried cooking at 131 F for two hours; of course I've also tried 72 hours(I began cooking sous vide expecting 72 hours to be the standard of excellence). The thing is that 100 percent of the 24 hour steaks eventually seem better than longer or shorter times.

I can immediately discount problems with the cooker temperature, with the post sear causing overcooking, with salt or seasonings. I considered and rejected the geometry of a large, flat piece of meat interfering with water flow, etc. from my perspective it seems as if 1/2 to 2/3 of round steaks and London Broils are prdisposed to taste powdery dry. I don't think this is about grain-fed vs. grass-fed cattle. If I could make all my round steaks taste like the 'good' group it would be miraculous.

I tried cooking a small bit of a London Broil along with my tri-tip meals (the only other meat I cook often at 131 degrees). The tri-tips came out perfect reddish pink medium rare, tender and juicy as always, and the ~6 oz. London Broil in the same water was always dry, whitish pink, with hard swallowing.(131F 24 hours)

Is this problem with round steak familiar to anyone out there?