Perhaps like some GR is better at creating ideas rather than implimenting them. Remember we all have strengths and weaknesses.
Actually it seems more like the other way around, a lot of the time. After all, he rewrote all the scripts in the first two seasons, but stopped doing so in the third, and the quality of the writing was much more solid in seasons 1-2. So that suggests, as Sir Rhosis
said, that he was good at the implementation of other people's ideas. But the scripts/stories credited to him, the ideas he came up with himself, were often pretty goofy or problematical.