In his commentary, Peter Davison (Tristan) remarks that this was partly due to the fact that they ran out of story ideas. (James Herriot actually had the same problem with his books). The series' creators had no idea the series would become so popular so instead of stringing out the James-Helen romance and saving some ideas, they stuck every incident from the first few books into the first season. Result: they had to invent and reuse a lot of material. Peter Davison made a wry remark about his character, who is supposed to be a flirt, getting older and older while the "bright, young things" kept getting younger and younger. Tristan starts out as an eccentric (a "debauched choirboy" is the description) but ends up rather dull. In the 4th season, they created Calum Buchanan to supply the eccentricity that Tristan supplied in the first two seasons. Unfortunately, Calum just makes Tristan and Siegfried look like old fogies. It's kind of sad, although I suppose it reflects real-life.
They also toned down Robert Hardy's character in the later seasons, which was a mistake. They did it because the real Siegfried, who by all accounts was quite the outrageous personality, expressed some disapproval. Hardy, who knew the real Siegfried, had based his interpretation of the character on that knowledge and had already softened Siegfried's rather manic personality. But he was told to take it down even further. It's a pity since--as James Herriot's son, Jim, points out in his biography of his father--the crazy Siegfried was most people's favorite character. I love Robert Hardy. (For those of you who want to place him, he is Sir John Middleton in Sense & Sensibility; he plays the minister with the pin-striped robe in Harry Potter.) He's one of those British actors who pops up all over the place.
In the 4th season, Carol Drinkwater got tired to playing Helen and left. I actually prefer her replacement, who I think looks much more like a Yorkshire Downs' wife. Carol Drinkwater always looked like she was about to fly off to the Riviera, which actually is what she did in real life (well, to France at least). The spark is missing however. (Gossip central: Carol Drinkwater and Christopher Timothy had a romance in real life).
Christopher Timothy doesn't change at all. He was really excellently cast, and James Herriot himself thought Timothy portrayed his personality the best (out of all the TV shows being made at the time). Timothy manages to capture that laid-back, good-humored, yet somewhat tense personality that made it possible for Herriot to get along with the Farnons but gave him ulcers later in life. (He also had an extremely poor money sense.)
The worst thing about the later seasons (although the 4th season isn't so bad) is that they decided to overlay every scene with this totally sappy music. I can't decide if it is an 80s thing or just a director thing. I think it is kind of an 80s thing. Scenes which are actually not played as maudlin come across as maudlin and in some cases, the sappy, trilling music is so loud it drowns the rather good dialog. I wish very much that when they had released the series on DVD, they had fixed the music, but maybe, they couldn’t. (Heaven help us if people actually like it; it's pretty horrible.)
All that said, I still recommend the series, all of it. All the seasons are sweet (even without the music), fun, very relaxing and you learn an awful lot about vetting in the 1930s to 1950s.