Depth is the biggest problem.
We returned 10 o-linemen this year and added Braun and 4 freshmen. Mack and Charlie both blew a hole in this position group and Herman is doing a decent job at fixing it, but it takes some time to develop quality.
We lost a really good 5th year senior LT, a pretty good 4 year starter at LG, and a senior RG. None of their replacements are a significant upgrade.
Youth is a factor with Cosmi and Angilau replacing Anderson and E Rod.
Kerstetter has almost everything you want in a lineman, he's just not as physically dominating as the elite guys.
Braun and Shack are both pretty good.
Beyond those 5 you have Okafor and Imade as primary backups and both are a drop off from the starters.
There's another factor that's affecting the o line, but I don't know if they're banged up a little or if it's coaching. But they aren't improving as the year goes along and they seem to be getting a little worse.