The acting profession is very complicated. You need a good match to your personality in terms of a screenplay. More importantly, you need a good agent.
I think of somebody like Rock Hudson lucking out particularly with the latter. He was able to avoid any scandal about his private life thanks to a great agent, Henry Wilson, with connections. Also his box office appeal had only fleeting declines. For example, he was at his peak in the mid-fifties, around the time Giant was filmed, then suffered a decline in the late fifties around the time of his curious divorce with lesbian Phyllis Gates. He then bounced back to the top thanks to Pillow Talk, a perfect example of how not one, but two, declining careers got re-activated by just the right script match-up... and Doris Day hadn't been doing well the past two to three years either. He pretty much coasted the sixties thanks to a few challenging parts that went against "type" like Seconds, then settled nicely as an action star in wartime vehicles before comfortably positioning himself on TV throughout the seventies and early eighties, even appearing on Dynasty in his final days. Thanks to proper management, he was well enough liked by everybody that his disclosure of having AIDS actually helped him more than hurt him in his final days in terms of publicity.
In contrast, you have somebody like Leonard Whiting. As a teen, he did an excellent "method" acting performance in Romeo And Juliet, which started filming the week of his 17th birthday in late June 1967. Today, it is still the most popular film version (although TCM has been reluctant to air it) more so than the later Leonardo version. Yet he was not lucky like Rock Hudson, nor did he have a studio grooming him in his formative years (like Universal did Hudson). Leonard's co-star Olivia Hussey at least managed to get supporting roles throughout the seventies (playing Mary in Jesus Of Nazareth on TV). I don't think she was necessarily a better performer than he was, but she had better control over the parts she played. Both had nice voices that rendered well for cartoon work, but, again, she did more of that as well than he did. He did have some minor success in music, but you can see how sparse his filmography is.