This once happened to Jim Brosnan when he was with the St. Louis Cardinals back in 1959. In a meeting with pitching coach Clyde King, Brosnan complained that neither of his fastballs was working for him. King called in a catcher and had Brosnan do some throwjng for about fifteen minutes, and then told him to drop that four-seamer---his two-seamer was working much better for him. So Brosnan became a sinker-slider pitcher and was very successful for some years. You might consider this---develop a two-seamer which in effect is a sinker, and add another pitch, perhaps a slider or a splitter---you would do very well with them.