People love to pile the putrid 3rd down % on the QB alone. You have to go all the way back to 2011 to find a Miami team with a 3rd down conversion rate above 40%. Most years we were low 30's.
The main reason for the bad 3rd down rate, and why it was bad before Rosier was QB, and even before Kaaya, is poor performance running the ball on 1st down. When your average distance to gain on 3rd down is 7+ yards it cuts your playbook in half. People complaining about too many vert routs and not enough crossing routes, but don't realize how much the route tree becomes limited when you have to gain over 7 yards. It allows DBs to sit on certain routes, and also gives more time for the opposing d-linemen to disrupt things in the backfield.
It's been a long time since we've had a good o-line--we've had good individual performers, but not what you'd call a solid overall unit that can reliably get any significant push on traditional run downs, like 1st and 10. And 3rd down conversion rates have suffered because of it.
If you look at all the top teams in terms of 3rd down %, they have one thing in common--they run the ball well on 1st down.