Lots of things.
1) The rush hasn't been as good as needed at times.
2) Hawk and our other ILBs aren't tall enough. Part of what helps prevent passes over the middle are tall ILBs that force the QB to put more touch on those throws. More touch leads to more time for safeties and ILBs to pop the received and jar it loose. Or make a play on the ball.
3) Our ILBs aren't good enough cover men. Our corners are generally pretty good, especially when everyone is healthy. So backs and tight ends end up covered by 'backers. You could bring out more corners, but then the opposition will just ram it down our throats. So we get stuck doing things with our safeties that ends up (imho) predictable to stop the bleeding. This leads to 1-2-3 to work together:
A sub-optimal pass rush gives the QB to scan the field and gives the receivers time to get open. The ILBs are often beat and the safeties are trying to cover up other mistakes. Now someone is open--either an outside guy because he broke free after 5 seconds (exageration for humor) and is in single coverage because a safety is helping out with a tight end OR the tight end is open because the safeties are playing deeper coverage. Or we sent 5, leaving only a single free safety. Or we sent 5 and chose to ignore an under route in exchange to keeping 2 guys deep.
So what do we do?
1) We need more pass rush, but really, the entire NFL does. You can't have too much. Matthews and Daniels are just fine, keep doing what you're doing. Neal has started to really improve. Even Perry has looked good on the right side. Maybe Datone makes the jump next year. Defensive linemen have one of the hardest jumps from college to the pros. If those 5 play up to potential next year, we'll be fine (imagine all 5 on the field at the same time!), but you always need more depth.
2) Find that super-star ILB. Easier said than done, but he would solve 2 or 3 other issues at the same time.
3) Replace Collins. Even easier said, even harder done.
And of course, we haven't fixed our run defense, which will take a hit next year with the potential losses of Raji, Pickett, Wilson, and Jolly. All are free agents and it is very unlikely we can or want to keep all of them.