Low oil would be my guess too.
Remove the moco unit using a big (24mm?) socket.
Pour the oil out into a jug.
Have you got enough? If not then that’s probably the issue.
Then get the right amount of oil and pour it in, then replace the damper unit.
It’s a 2 minute job (assuming the forks aren’t fitted to the frame yet).
Otherwise (and this is tricky to describe) make sure the lever and ‘the paddle type thing that closes the damper’ are lined up correctly. So the damper really is moving from fully closed->partially open->fully open when you rotate the lever on top of the damper unit.
I sold some Rebas once and during a DIY service the bloke I’d sold them to managed to bork them up so the lever and the ‘gubbins’ were out of sync. Because the lever only rotates through about 200-250 degrees and the lever wasn’t lined up right he was turning the lever from Closed to Open, but the damper was only moving from partially closed to slightly less closed.
Like I said, it’s hard to describe, but rotate the lever while looking at the bottom of the damper unit and it shoul dbe obvious when the holes go from uncovered (open) to partially covered to completely covered (locked).
