There was some discussion of Tom Clancy's work last night on twitter, and whether it was his right wing politics that made his latter books bad. I would say two factors mattered as much or more for him and for many authors in this genre, which were then exacerbated by his right-wing politics.
The challenge for thriller authors, as opposed to mystery authors, is that one often needs some political dynamics to kick off and then complicate the crisis. And politics is, perhaps like sex, hard to write in a way that is semi-realistic and yet still interesting. It is no accident that Hunt for Red October is Clancy's best book--it has the least amount of political machinery required to move things along. Red Storm Rising has a modest amount of incredibly stupid politics--that Soviet response to the loss of some oil production facilities would be to invade Europe so that it could invade the Mideast--but these political dynamics are mostly out the way early.
The problem is that as the Jack Ryan series goes on, he gets deeper and deeper into politics, and politics is required to move things along more and more. And Clancy sucked at politics. Not just right-wing-ness, but just stupid, simplistic, often racist takes.
The second factor that kicked in mightily is that big name authors get edited less and less as their success gets bigger and bigger. Knowing what we know about Clancy, there is no doubt that as he became quite popular, his editors probably stopped doing much editing. This is something my wife, a freelance editor, has long believed about most successful authors, knowing what she knows from the inside of the business. I will always admire her big name clients for wanting her to work on their stuff--that they wanted to write the best they could and knew that more help was better than no feedback.
Anyhow, Clancy (and others) could not write politics, and Clancy (and others) became less edited over time. Once Clancy made Ryan President, the politics became ... unintentional humor. And I stopped reading Clancy.