AFB and Jurbo are the exceptions IMO. Both have only improved and are consistently as good as people rate them to be.Interesting you mention Campbell-Gillard.
Personally, I am seeing a bit of trend in the game at the moment with young props. They seem to burst on the scene and have a couple of bumper seasons before all the enterprise goes out of their game. The same thing happened to Klemmer. He used to be explosive and dynamic (much like Campbell-Gillard) but a couple of years of first grade, they become plodders. They amble up to the line and sure, they work hard and make some metres, but all the fizz has gone out their games.
I don't know what to put it down too. I think maybe weights programs have a bit to do it. A lot of outside backs look lightening fast during their first 1-2 seasons, before putting on lots of weight and losing their speed. I noticed it with Turbo and Walker...they were lean and quick when they first started out in FG and although still fast, they are a yard slower to my eyes.
Regarding backline players, injury probably plays a big part but I think players like Turbo and Walker realise they need to rely on more than just pace. Both were able to develop their game beyond speed as did Snake. For that reason, he's probably one of the most crucial signings for us this year.