From what I've read over the past couple-three years, evolutionary developments in popular college offenses have resulted in college OL (especially IOL) not being trained, as much as they once were, in the types of reads and techniques that are required in NFL offenses. Similar thing with college TEs and blocking techniques.
So, it often takes more time for college prospects to "get it" (and even longer if they don't have a good OL coach when the arrive in the NFL). Those that do, eventually, learn what they're supposed to do, aren't necessarily the guys with the athleticism to be able to execute consistently.
And, then, there's the scheme-fit variable, too.
Kline has apparently been able to gradually build on the "primary school" education he received from Scar and become more consistent. Devey may not have the functional athleticism to execute consistently well on the OL, but he has size and apparently has figured out most of what teams need him to do, so he keeps getting employment as a backup, especially since the guys who are good enough to start are relatively rare.
The blocking thing is also why TEs who can actually block well, even if they're mediocre receivers, are still getting 3-year/$4M APY contracts at age 32, like Darren Fells.
