Max,

If you have all linkage, verify everything is adjusted in a neutral position. This will allow shifting equally in both directions. If you have a cable and linkage, again do the same. Also, make sure the cable not stretched. Should everything check good, open the control valve and inspect all springs, valves and bores. Broken springs can cause problems. Valves have broken in two and bores can get scored restricting movement. Good Luck.