If you look back at the D-Line recruits for the 2009-2013 classes, it's easy to see why the defense is struggling so much.
2009
Peck 3* DE - Serviceable player, should probably be a backup.
Chrichton 3* DT - Washed out of program

2010
Afo 3* DT - Washed out of program
Kanuch 3* DT - Switched to O-line
Tuiloma 2* DT - Serviceable player
Kafusi 4* DE - Moved to OLB for incomprehensible reasons
Rowley 4* DE - Shaking off mission rust

2011
Olomua 2* DE - Washed out
Kafusi 2* DE - Outgrew football
Lesatele 2* DE - On a mission?
Reden 2* DE - Moved to TE
Tausinga 3* DT - On roster, injury issues
Kaamatule 3* DT - On a mission?
Pritchard 3* DE - Transferred

2012
Taamuta-* DE - Washed out
King -* DE - Can't find the field
Johnson 2* DT - Serviceable player

2013
Laulile 2* DE - Seeing time
Pulsipher 2* DE - Mission

That's 19 guys from the 5 classes that should be stocking our current D-Line. Of those guys, only 6 are on the roster and playing D-Line, (including King, who doesn't actually play). Of those 6, you have 1 4* (Rowley), 1 3 star (Peck), 3 2 stars (Tuiloma, Laulile, and Johnson), and a guy who wasn't evaluated (King). We have done a terrible job landing talented D-Linemen, and a bad job developing the ones we do get. Fixing that has to be a priority going forward. The 2014 class had some decent prospects in Leaitaua, Dawe, Nacua, and Fuga, unfortunately all of those guys are at least another year away due to missions. The 2015 class has some really good prospects as well (Hunt, Tafua, and Mounga), but I think we need to bring in a couple of JC guys if we are going to see immediate improvement. Otherwise, next year's D could struggle just as much as this year's.