Few people admire Harrison Ford more than I do. Just that fact that he played Han Solo will make him a favorite of mine for the rest of my natural life. For nearly 20 years, Ford appeared in rock solid films that enjoyed critical acclaim and HUGE box office success. As an actor he played just about every genre of character with brilliance, from action heroes (Indiana Jones, Blade Runner, Star Wars) to dramas (Regarding Henry, Witness, The Mosquito Coast) to comedies (Sabrina, Working Girl). Ford has done it all (with the exception of playing a good vilian, something I'd like to see him do someday). Just his presence alone in a movie would bring people the theater in droves.
Then, after Air Force One in 1997, something unexpected happened. Ford films started to decrease in quality, and people stopped going to films just because he was in it. The slide started mildly with the disappointing (not outrageously horrible) Six Days Seven Nights, the film that Harrison started to show his age. After that came one of the worst films of the 90's Random Hearts, a film which Ford reportedly called "a huge waste of time". What Lies Beneath was just a terrible horror flick with more cliches than a Scary Movie film. Although not a total waste, K 19: The Widow Maker was a mild let down, and then the much marketed Hollywood Homicide completely flopped at the box office. That's 5 films in a row. Only an actor with Harrison Ford's status and reputation could survive a streak like that.
However, even an actor with Ford's credentials only gets so many chances. If you keep disappointing the paying audience film after film, the audience will at best lose interest, and at worst begin to proactively avoid you.