Before Shawshank, Stephen King was box office poison.
Misery, Carrie, Stand By Me, Pet Semetary, Children of the Corn, Cujo and the Shining all came out before Shawshank. Granted he had some flops to, but he was very far from being “box office poison”. If anything, post Shawshank movies made from his works have been a lot more hit and miss then those before.
(and Shawshank itself was actually something of a flop, at least as measured by the boxoffice take)