I recall Golic saying that when you come into the league you share a bond with your team just like High School and College. Once you go to another team it is tough to recreate that same feeling with a new team. Plus when some guys get the big money it changes their motivation, which combined with the loss of chemistry, is why so many free agents don't play as well as they did with their first team.
In other cases the free agent is getting older and his skills are declining and that does not help with the loss of chemistry.
If a team signs a free agent it should either be a superstar like Reggie White, who is a leader wherever he goes and is self motivated, or is a lower risk type.
The chemistry thing is very big according to him though and is why many new coaches like to get players that they are familiar with, in hopes that they feel the comfort level with many of the same coaches.
I hope the Rams can sign their own and if they sign a WR it is a guy that can succeed no matter where he is, like Brandon Marshall.