Try to adjust chains below the motor cover may be a chain switch be activated and see if some of the cover switches are activated also. if somethink like that occurs the lower function remain deactivated. try those and will see what next.

sounds to me that you have a gate switch problem which will cause the issue you describe bearing in mind that you checked bypassed the lowering button and that you have done a continuety check on mast cable wires

Hi, have recently experienced similar issues.
How did you get on repairing?
1 st incidence we was due to oil not coming back out of lift cylinder until we removed the velocity fuse / anti drop valve / safety valve.
Replaced the velocity fuse and truck appears ok.
This is the first time I have come across a velocity fuse that stops the descent / oil 100%. This I believe explains why the platform will no lower at all once the platform is at 100% lift height and the velocity fuse has tripped, (line pressure holding valve shut).
Previously all velocity fuses on forklifts I have seen have a controlled lowering rate once the velocity fuse is tripped / activated.
2nd incidence of wave stuck in the air is happening today, see what results??