Improving defense - what am I missing

[joey2000]

I have what I thought was a pretty good/not great D built up. My starters all had at least a "good" rating (some better) except for some "average" LBs and FS. Long story short my D mostly sucked (and mostly stayed healthy). I mean like near bottom of the league sucked. wth? Are LBs and FS that much more important? I would have thought DEs and CBs were. Do I have to have a really high rated person somewhere vs solid across the board?