Continuity mistake: When Nicole takes Whitfield for his first ride in Herbie, the car rears up onto its back wheels. But in the very next shot of Whitfield's startled reaction, the rear-screen projection of the street behind them is footage taken from a level view, not an elevated one. (00:15:50)
Revealing mistake: When Herbie dives into the ocean, water can be seen coming out of the open back window, even though the next shot shows the inside completely dry and the window closed.
Continuity mistake: During the flashbacks to Herbie's racing days, his front license plate disappears and reappears throughout the final racing sequence. (00:22:00)
Continuity mistake: When Herbie is next to the surfer on the sea at the beach, in a close-up Willoughby and Nicole are sitting in the car, but in a full view the car appears completely empty.
Continuity mistake: When Nicole takes Willoughby for a ride in Herbie for the first time, in the rear window it's clear the car is travelling through the streets of San Francisco, but in the previous shot the car was travelling through streets with no houses on either side.
Continuity mistake: When Herbie is taking Nicole and Willoughby to the beach, Nicole is driving and Willoughby is in the passenger seat. When we see Herbie turning towards the Golden Gate Bridge, you can see Nicole (wearing a powder blue top) in the passenger seat. This is a recycled shot of Herbie driving Nicole and Willoughby to the chicken tournament earlier on in the film.
Continuity mistake: When Mrs Steinmetz activates the window cleaning machine to fill Mr Hawk's office with soap and water, Mr Hawk grabs hold of Willoughby and you can see Willoughby is covered with soap. When Willoughby runs after Herbie a little later on, he now appears completely clean.
Audio problem: When Herbie is pushing furniture on to the two security guards at the warehouse, the first security guard yells "Look out, Smitty! Run!" but his mouth does not move.
Audio problem: Pay phones are programmed to drop the coins into an internal storage box after the receiver is placed back in the cradle. But at the airport, when Whitfield hangs up on his mother (and he didn't call collect), there's no sound of coins dropping. (00:38:00)
Answer: I don't know specifically why these particular actors did not return. There are any number of reasons why an actor does not reprise a role in a sequel: salary disputes, creative differences, scheduling conflicts, an actor wishing to avoid being stereotyped, and so on.
raywest