I always thought that it was the same wrighters right through. Grant & Naylor ?
The problem with a series like that, is that the first six were written almost as one continuous project. And then they stopped.
When they tried to revive it, after a break of what? about five years? It was never going to be the same. By then it had just lost it's zest.
The complete last couple of series, had about the same chuckle factor of just one episode of the early stuff.
Also a factor, was the fact that in the early days, all of the actors were up and coming unknowns, for the most part. It was fresh, and they had something to prove. They were still a bit raw, and it showed, and came accross as a group of mates having a laugh.
By the later series, they were seasoned actors, and it showed. It had lost a lot of that raw-ness and cheekyness about it. The gags were more obvious and telegraphed.