It never made sense that the co-writer of The Office (Merchant) was not involved at all in the David Brent film (surely this was part of the franchise). The pair do not follow one another on Twitter, and any interview that Ricky Gervais does there is absolutely no mention of Stephen Merchant, and I have watched a lot of these interviews. What is up with them? Something has happened and it ain't that old chestnut of ‘scheduling’ issues.
Just because you make money with someone and work with them does not mean your going to best of friends. I am not close friends with anyone I work with outside of work, but we all get on in the workplace.
Also don't forget that Stephen Merchant has done things on his own which Ricky was not a part of, as has Karl Pilkington, so it works both ways.