Is your locking piston moving all the way back out?
with the steering wheel pushed in, my locking piston extends about 4mm, with the steering wheel fully extended, it's almost 8mm.
Steering wheel locked at short telescope:
Steering wheel locked at longest telescope:
Try spraying it with contact cleaner while moving the lock-lever. It shouldn't need any lube.
Also make sure that the block/wedge that the piston pushes on moves freely when the lock piston is retracted. Again, contact cleaner is probably the best fix. If all that's good, it has to be a problem inside the tube.
I have two spare steering columns that I don't expect to ever use. Let me know if you need one or parts of one.