Many possibilities. Not much to narrow the solution. Is this the only TP that you loose? Are the other 91W transponders at their normal Q?? Some possible causes: Dish not stopping at the same spot?? Bumping the dish one way or the other gets it?? Try going to 97 from the east(<91), then from the west(>91). {One way gets it, the other - not?} Or maybe lost a count or two?? Drifting L.O. ?? Try changing the TP freq + up to 2 or 3 Mhz.
Could be the dish isn't tracking as good as it could be. What 's your equipment list??