AMC gear box

Might be that you are unfamiliar with it. With engine off, without pulling clutch and rear wheel off ground. If you can go through all gears, no problem. You might need to rotate a bit on the wheel for the dogs to mesh.
Then check clutch operation. Bike on wheels. If you can freely move the bike when in gear with clutch drawn everything seems to work.
When riding and want to downshift, pull clutch lever, give a little burst of throttle and then shift down sometimes helps.

The two items that commonly affect changes are the Pawl spring which needs to be just clear of the pawl and a draggy clutch which can have many issues and has been the subject of many posts. Try removing the gear position indicator and its bolt as they can bind the shafts together.

The Mick Hemmings dvd is well worth the investment, buy it now and then if you do need to work on the box, you'll already have it to hand. 

Primary chain too tight,
Pawl spring incorrectly set or split.
Gear lever return spring stop plate worn. 

 Your statement 'when I pull the clutch in the gearbox does not free up'.  Begs the question, Is the clutch releasing properly when the lever is pulled? 

Gear changes heading one way but not the other are often a sign of one of the two selector springs needing changing.
They both sit on top of each other inside the gearbox by the Gearbox Selector Quadrant.
The thin selector arm can go weak, distort or may have been incorrectly fitted.
The chunkier Return Spring is prone to cracking or even breaking apart.
My money would go on the latter.