It may be as simple as the linkage, all the linkage must be free from binding and able to return without any friction. Also if one adjusting rod is slightly longer/shorter than the other that could be your issue. Make sure you have strong return springs and disconnect the throttle cable and manually actuate the shift linkage and see if the throttle-stops rotate at the exact time. Also make sure your throttle cable is not binding anywhere and sccles smoothly.