[Closed] Faulty i900 dropper post

Here's what it does (or doesn't do)

1) Pull the lever and drop the post.
2) Release lever to lock at full compression.

All good so far...

3) Pull the lever to raise but here's the problem, it only lifts about 30mm and the stops with the usual little thud you get when you get to full extension.

You can pull the saddle (with the lever closed) and get the post to fully extend but it sucks back down when you let go (as you'd expect)

I've stripped it back to remove the inner post / piston assembly and it does the same thing so appears to be a fault with the actual piston itself rather than the outer post / bearings.

Any ideas?