I think the notion that defenders can't get off blocks is a bit misleading because most offenses have moved to nickel-and-diming our defense; meaning they are mostly attacking it with quick west-coast offense passes. If a normal play lasts 7 seconds and the defended is engaged in the first three passes by an OLinemen, but then comes free for the rush, then that defender looks like he has shaked the block. But if the play lasts 3-4 seconds with the QB getting a short throw off, then it looks like the defenedr has been engaged the whole time, unable to come off the block. But nothing has changed for those first 3 seconds, and this is why it "appears" our defenders can't come off blocks.
The real question then is, how are offenses so successfully nickel-and-diming us? The answer is simple. With the loss of Newman and Pacman, Dallas has been forced to play off the ball, as to not give up the big play with subbed-in rookies. So offenses have responded by nickel-and-diming with success.
I just hope Dalls does not play their corners off now that Newman is back; that's becasue that's what they did the last time Newman and Pacman were out there against Washington in the first game. Then, Dallas played them both off and the Commanders had success nickel-and-diming the defense. Now, it's no shocker that any "prevent" scheme gives up yardage. At least with Newman and Pacman back there the defense gave up fewer points because they are savvy veterans (remember, Washington had 2/3 of the time of possession and only managed to come away with 22 or so points --- nowhere near the blowout you would otherwise expect a team to have if they held the ball that long). But that scheme has unraveled with Newman and Pacman out of the lineup (look at the Rams game; the Ginats game -- where the opponent's extended time-of-possession has resulted in higher points).
So with Newman back in the lineup, Dallas must - and I mean, MUST - press the WRs on the line of scrimmage, otherwise the defensive problems will continue, more time of possession will be given away, and that will yield less time fro Romo to do his thing.