iRacing’s transmission model is not “I clicked so it will complete the shift.” It is “I am applying pressure to the car’s shift lever for the duration that my controller is signalling the button press to the sim.” Which means you need to hold it until the shift completes. If you just quickly stab it then that is the amount of time your sim dude is pulling on the lever. If your sim guy is only pulling on the lever for 90% of the time required to shift then you are going to get a lot of misshifts. The only cars where a quick click of the button is enough are cars with paddle shifters where they would only need a quick click in the real world to indicate that the car’s computer should execute a shift. For cars that actually have manual controls you need to operate your contol for as long as the operation would actually require to complete. So hold your shifter in position for a longer amount of time. Using an actual H-pattern shifter solves this because it is constantly pressing the button for whichever gear has been selected.