It always appeared to me like the entire cast just phoned it in in Nemesis. They weren't fit, they looked fat. And tired. And worn out. And lazy. Stewart sounded like he had a cold.
Compared to First Contact where they seemed at the height of their fitness and performance. Frakes seemed to be at his thinnest in First Contact, Dorn at his strongest. The perfomances were alround enjoyable, the chemistry was wonderful.
Insurrection was a bit "lazier", but still about the same level.
But Nemesis... seriously, something went entirely wrong there. There was a 4 year gap between the movies, but on screen it seems to me like 10 years.