What tranny? 3sp, 4sp? Does it have linkage or does the shifter go into the top? If it has linkage it is probably a pivit bolt or other link component. Take off tranny floor cover and unhook shifter linkage and see if you can shift it by hand/wrench to be sure it's not an internal thing. If the linkage lets the other gear positions get out of "neutral", it won't shift. They all have to be "timed" using adjustments. Don't know what "shifter" you put on but they have a pull up feature for R and that can malfunction also. Need specific info from you about shifter and trans.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn