Transmission And Shifting Gears

I have a 1977 Dodge Aspen 2 Door Coupe that I have been restoring for the last six months. The other day ago I ran the vehicle for a test run. The transmission will go into all gears manually but when driving it only wants to stay in first gear. It doesn't want to go into second or third gear. I have looked on the top of the engine and on the bottom of the trans to see if anything has came off but I see no issues of any kind. Any advice would be greatly appreciated. I have no manual to refer too. Thank You.
Author: Aspen77