Some good discussion on this page on the defense. One of Babich's favorite lines in PCs this year as reason for defense not performing is some variation of "the rush is not in sync with the back end". It seems he says it every week. I don't know why he can't fix it.
To go along with that, as has been mentioned, even when the play call is good (and presumably the 'rush is in sync with the back end'), guys are just missing tackles. After the Dolphins game I saw an analysis of a bunch of the 3rd and long plays that were given up and in each case the call worked perfectly (corner blitz, simulated pressure etc) and in each case some guy was right there to make the play and whiffed. One thing Babich and McD are always careful to do is never throw an individual under the bus and when asked about a play that later is revealed to be a simple poor individual tackle attempt etc they always describe as an 'execution' issue
I'm also starting to get a little skeptical on Babich - esp when every week he says the same things about getting off the field on third down, gap integrity, 'rush in sync with back end'. My hope is that with people out and rookies playing etc the defense we are seeing now is not the one we will see at the end of the year and in the playoffs (one thing that gave me hope in preseason was Babich mentioning the defense needing to get off the field quick this year to get the ball in Josh's hands, which seemed to indicate a more aggressive approach - but we haven't seen it so far)