No kickdown gear in a TH350 transmission?

I have a 305 engine with a TH350 transmission in my '79 Firebird. It does not want to go into ping gear. What can the problem be and how can I adjust it to work properly? I have replaced the vacuum modulator, filter, and fluid. It does not slip or have any other problems. It shifts perfectly fine through all other gears. What can the problem be and how can I solve this?